Leaving the European Union would threaten jobs and put the UK's economy at risk, leaders of some of Britain's biggest companies have said.

Bosses - including those of BT, Marks & Spencer and Vodafone - signed a letter published in the Times, saying an EU exit would deter investment in the UK.
Leave campaigners point out two-thirds of FTSE 100 firms, including Tesco and Sainsbury, did not back the letter.
A referendum on whether the UK should stay in the EU will be held on 23 June.
David Cameron earlier took questions from employees of mobile phone giant O2, one of the signatories of the letter, at its headquarters in Slough, on the first of a series of tour events to sell his pro-EU message to voters. The move is designed to reach beyond Westminster and the emerging divisions within the Tory party.
The Mayor of London Boris Johnson, the most high-profile politician to back EU exit, has urged people to focus on the issues and played down talk of ill feeling between him and the PM, saying "team spirit" within the party was good.
In a move described by No 10 as "unprecedented", chairmen or chief executives of 36 FTSE 100 companies signed the letter, organised by Stronger in Europe and Downing Street, backing the campaign to stay in the EU, including Burberry, BAE Systems and EasyJet.
The FTSE bosses were among a total of 198 signatories from the business world, including the chief executives of Heathrow and Gatwick airports.
However, nearly two-thirds of the UK's largest publicly listed businesses did not sign, including RBS and Barclays.
Asked about why the majority of FTSE 100 companies did not sign the letter, Mr Cameron said companies were often reluctant to "make any form of political statement".
But he added: "If the leave campaign could produce 35 business leaders of this sort of stature they'd be over the moon and I don't think they have the prospect of doing that with FTSE 100 leaders in any way."
On Boris Johnson's decision to campaign against him, the prime minister said he had "huge respect" for the London Mayor and he had "great future in British politics" but added: "He has got it wrong on this one."
Bank of England governor Mark Carney, has meanwhile, said policymakers are "not making a judgment" on the outcome or consequences of the referendum.
Mr Carney told MPs on the Treasury Select Committee that the Bank was treating the June vote as it would "every other political event".
Analysis by the BBC's business editor Kamal Ahmed
For politicians supporting Britain remaining in the European Union, the blessing of business leaders is seen as a positive step.
And the list signing the pro-European letter - at least in part orchestrated by Number 10 - are certainly a hefty bunch. But a few health warnings.
First, many of the people who have signed have long been public advocates of Britain remaining in the EU. These include Sir Roger Carr of BAE, Iain Conn of Centrica, Dame Carolyn McCall of EasyJet and Bob Dudley of BP.
Second, some of the business leaders named say that although Britain would be better off economically in the EU, the UK is still a significant market.
Read Kamal's blog
The letter from business leaders said Mr Cameron had secured a commitment from the EU "to reduce the burden of regulation, deepen the single market and to sign-off crucial international trade deals".
The signatories wrote: "Business needs unrestricted access to the European market of 500 million people in order to continue to grow, invest and create jobs. Britain will be strong, safer and better off remaining a member of the EU."
John Holland-Kaye, the chief executive of Heathrow airport, denied he had been pressurised into signing the letter, saying his company wanted to get the message across that the EU had "opened up the aviation market and reduced the cost of flying".
"Lower costs of flying mean we can go on holiday more easily, we can go on business more easily - so whether you are going on a stag night or exporting there is a real benefit of being part of the EU," he told the BBC News Channel.
Richard Tice, co-founder of Leave.EU, said Downing Street had admitted using taxpayers' money and "applying pressure" on FTSE chairmen and chief executives to sign the letter.
"The truth is that despite the bullying of a prime minister who has no real business experience, it is other normal commercial factors which will determine the continued success of British businesses to invest and grow," he said.
And Vote Leave, another group campaigning for EU exit said it was "disappointing to see the prime minister resorting so quickly to trying to scare people into voting for his deal".
A BT spokesman said: "Any suggestion that our company has been pressured into publicly supporting continued UK membership of a reformed EU is untrue" and it wanted to inform the public about how the EU benefits its business.
Find out where your MP stands
Meanwhile, senior figures from both sides of the debate are making their voices heard.
Boris Johnson urged people to ignore those "spreading alarm and anxiety" about the economic risks of EU exit, saying many of those involved supported the UK joining the euro in the 1990s.
"We have a great opportunity now to strike new deals for Britain to be the hub of new trading arrangements around the world," he said.
Former home secretary Alan Johnson, who is heading Labour's In campaign, has warned that two thirds of British jobs in manufacturing depend on demand from Europe and leaving could put up to 50,000 apprenticeships at risk.
"There is no progressive case for Britain to isolate itself," he said in a speech in Bristol. "We need to be leading in Europe, not leaving."
According to BBC research, 142 Tory MPs will campaign to remain in the EU, 120 to leave, while 68 have yet to make their positions clear.
Former Conservative leader Lord Hague has warned that splits over Europe in his party could take "generations to heal".
These divisions were laid bare during a debate in Parliament on Monday, when a succession of Tory MPs questioned the substance of the PM's agreement with EU leaders - announced on Friday.
The overwhelming majority of Labour MPs back continued EU membership as do the SNP, Plaid Cymru and the Liberal Democrats but the Democratic Unionists and UKIP are opposed.


A man who died after his motorbike collided with a car in Aberdeenshire has been named.

Frank MacDonald, 44, from the Brechin area, was between the B976 South Deeside Road at Strachan and the B974 Cairn o'Mount road at about 14:00 on Saturday when the crash happened.
The vehicles involved were a blue Suzuki GSXR motorcycle and a grey Toyota Hilux.
Relatives said the "much loved" father-of-three would be greatly missed.
The unclassified road was closed for more than eight hours following the crash. Police have appealed for witnesses to come forward.


An Australian aid worker who was freed by al-Qaeda-linked militants in Burkina Faso has vowed to continue with her medical charity in the country.

Jocelyn Elliott, 76, was released over the weekend after three weeks in captivity.
She and her husband Ken, 81, who is still a hostage, had provided services in the town of Djibo since the 1970s.
Militants reportedly said the kidnapping was an attempt to secure the release of imprisoned fighters.
Africa Live: BBC news updates
Profile: Al-Qaeda in North Africa
Mrs Elliott arrived in Burkina Faso's capital, Ouagadougou, where she met president Roch Marc Christian Kabore, on Monday.
"I really hope to be with my husband soon so that we can again go to Djibo and continue (our work) there," she told journalists.
She said she was happy to be with her "Burkina family" and thanked the governments of Niger, Burkina Faso and Australia, but gave no details of her captivity or release.
The aid worker was freed in neighbouring Niger after the jihadists from the group al-Qaeda in the Islamic Maghreb (AQIM) said they did not want "to make women involved in the war".
Niger acted as a mediator to secure her release, officials said.
The couple were kidnapped in Djibo, near the border with Mali, on the same day as a deadly attack on a hotel in Ouagadougou, which was also claimed by AQIM.
Burkina Faso's Foreign Minister, Alpha Barry, told the AFP news agency that "no ransom was paid or conditions imposed" by the kidnappers for her release.
Efforts to free Mrs Elliott's husband were continuing, he said.
In response to their abduction, local people in Djibo launched a social media campaign calling for their release.


South African Tokyo Sexwale ended his campaign to become the next Fifa president during his speech ahead of the vote in Zurich on Friday.

The 62-year-old told the Fifa congress: "I suspend my participation. I leave only four people."
Gianni Infantino, Sheikh Salman bin Ebrahim al-Khalifa, Prince Ali bin al-Hussein and Jerome Champagne remain.
Sexwale was a rank outsider and did not have the backing of the Confederation of African Football.
In January, Caf announced its support for Asian confederation president Sheikh Salman bin Ebrahim al-Khalifa.
Sexwale, a former prisoner in an anti-apartheid jail with Nelson Mandela, conducted a low-key campaign and there seemed to be a real chance that he might not win any of the 207 votes in the first round.
He said he was prepared to work with any of the four remaining candidates to replace Sepp Blatter.


We asked England's women your questions ahead of their next qualifier for the UEFA Women's EURO 2017 championship.

8 April 2016 Last updated at 07:35 BST
Fara Williams is the England squad's most-capped player, which mean she has played the most international games for the squad and Karen Bardsley is goalkeeper.
With two wins under their belts in the Euro 2017 qualifiers already, England's women are preparing to face Belgium tonight.
Watch the video to check out their answers.


Picasso, Turner, Van Gogh... Messi, Neymar and Suarez.

Barcelona's trio have transformed "normal life into art", according to Arsenal manager Arsene Wenger, whose side bowed out of the Champions League as expected after a tough lesson from the Catalan grand masters.
Neymar opened the scoring with a light brushstroke, Luis Suarez then splashed a bit of colour before Lionel Messi revealed his latest piece, entitled 'A Deft Dink Over Ospina'.
They were works of art, so we thought we would pay tribute to them with our own dedicated gallery:
Let's compare the 'No 5, 1948' by renowned United States artist Jackson Pollock on the left, which was sold in 2006 for £115m, and the 'MSN Mix' from Barcelona's 3-1 win over Arsenal.
The Opta masterpiece alongside the Pollock shows the passes, touches and areas covered by Messi, Suarez and Neymar in Wednesday's win.
The Barca trio are estimated by transfermarkt.co.uk to be worth 300m euros (£232.5m), which makes Pollock's painting appear rather... cheap.
This painting, translated into English as 'Goals: Galore' shows how many times Messi, Suarez and Neymar have scored this season so far. The total of 106 is exactly the same number of goals that bitter rivals Real Madrid have scored in total in the Spanish league and Champions League combined.
Argentina's finest footballing export since Diego Maradona has now scored 498 career goals for club and country - two away from the magical 500.
Who will Messi score his 500th against?
They score goals as a trio, but they also help each other out as a collection. And they're even best friends off the pitch. Life imitates art, as the saying goes.
This piece, entitled 'To Me, To You', shows Suarez (14), Neymar (14) and Messi (9) have assisted each other's goals 37 times this season. Manchester United have scored 37 times in their entire Premier League campaign so far.
Our final piece shows Arsenal manager Arsene Wenger (right) alongside Norwegian artist Edvard Munch's famous The Scream of 1893.
This 'Je Scream' is the reaction Wenger probably gave when he realised his side were drawn against Barcelona.
It was also surely the emotion he showed after Messi scored his ninth goal against them - the most by any player against a single opponent in the Champions League.
"We must admire art and they have two or three players who transform normal life into art. I respect that and I believe it is pleasure as well," admitted the Frenchman. "For me, is it suffering."


On the night of 31 August, Mandya, a district on the banks of the fast-depleting Cauvery river in the southern Indian state of Karnataka, received heavy rains after three weeks of unusually dry weather.

The rain-lashed night brought hope to local farmers like Shekar Guruve Gowda of Melkote village who grow crops such as sugarcane, maize and finger millets.
Next morning though, the sun was shining again. Mr Gowda, whose rain-fed farm is a mile away from his home, spent about seven hours sitting under a coconut tree waiting for more rains, which would help his crop to survive.
Two weeks on, he is still waiting for rain, and Mr Gowda now worries that his crops will die.
Although Melkote is surrounded by lakes connected to the Cauvery through canals, they have been running dry because of lack of rains and insufficient water in the river.
Apart from feeding six family members, Mr Gowda also has to provide fodder and water for two cows and two calves.
"I have not been able to grow rice even for my family. I depend on cheap state rations for food now. As a farmer, if I have to depend on someone else for food, it's a worrying situation for me and for India," Mr Gowda said.
The Cauvery, which flows through Mandya, is at the heart of one of India's intractable water disputes.
Earlier this month violence broke out in Bangalore, the capital of Karnataka, and other parts of the state after a Supreme Court ruling ordering Karnataka to share water from the river with neighbouring Tamil Nadu. Both states say they urgently need the water for irrigation and a battle about access to it has raged for decades.
About 40 miles away from Mr Gowda's farm, in Kirugavalu village, Sayed Ghani Khan, a farmer who grew paddy on a 10-acre plot last year, is waiting for water so that he can cultivate at least an acre of land this time. Half of his land is irrigated; and the other half fed by rains.
Mr Khan, who has received government awards for excellence in farming, says he is worried after a local official told him that no water from the Cauvery would be available to irrigate farms this year. The lack of sufficient rains has worsened matters.
The majority of the 30,000 villages in Karnataka received less than the required rainfall in August. The situation has not been very different this month. Some 41% of the 3,598 irrigation tanks in the state are dry.
Only two thirds of the cultivable land in four districts straddling the river - Mandya, Mysore, Chamarajanagar and Ramanagara - were cropped until 12 September, say officials.
Water levels of four reservoirs in the Cauvery were at a fourth of normal levels, and half of 2015, according to officials. Whatever is available has to be shared.
"The south-west monsoon that brings 70% of the country's annual rainfall, which was weak here until now, is set to end soon. And we see no signs of improvement. Farmers are in dire straits. People have to use water judiciously and efficiently. Else, we will have a drinking water crisis in summer," Karnataka State Natural Disaster Monitoring Centre chief Srinivas Reddy said.
When the Supreme Court on 2 September asked Karnataka to "live and let live", the state softened and offered to release 10,000 cubic feet of water per second to Tamil Nadu every day for five days.
On 5 September however, the top court ordered Karnataka to release 15,000 cubic feet of water per second for 10 days. This ruling was later modified to 12,000 cubic feet of water per second until 20 September.
Mandya, where the farm crisis is looming large, became the epicentre of protests. Vehicles were torched and roads were blocked. About 37 people were arrested.
The violence in the technology hub, Bangalore, forced the closure of offices and disruption of public transport. According to Bangalore police, a 22-year-old-woman led the mob and set fire to 42 buses on the outskirts of the city. Police have imposed an emergency law that prohibits public gatherings, and more than 15,000 officers have been deployed across the city.
On Tuesday, the Supreme Court directed Karnataka government to release another 6,000 cubic feet of water to Tamil Nadu for seven more days.
Karnataka's Chief Minister Siddaramaiah has said that the order was "unimplementable" and the water was only enough to meet state's drinking water requirement until May 2017.
Karnataka has now decided to defy the Supreme Court order and not release water to Tamil Nadu.
The decision is likely to trigger a constitutional crisis and bring more worries to the two beleaguered states in the days to come.
Prabhu Mallikarjunan is an independent journalist based in Bangalore


A paralysed man has regained limited use of his hand after pioneering surgery to bypass damage to his spinal cord.

His injury meant his brain could not "talk" to his hand, meaning all control was lost.
Surgeons at the Washington University School of Medicine re-wired his nerves to build a new route between hand and brain.
He can now feed himself and can just about write.
The 71-year-old man was involved in a car accident in June 2008. His spinal cord was damaged at the base of the neck and he was unable to walk.
While he could still move his arms, he had lost the ability to pinch or grip with either of his hands.
The nerves in the hand were not damaged, they had just lost the signal from the brain which told them what to do.
However, the brain could still give instructions to the upper arm.
The operation,
          described in the Journal of Neurosurgery
, rewired the nerves in the arm to build a new route from brain to hand. One of the nerves leading to a muscle was taken and attached to the anterior interosseous nerve, which goes to the hand.
Ida Fox, an assistant professor of plastic and reconstructive surgery at Washington University, told the BBC: "The circuit [in the hand] is intact, but no longer connected to the brain.
"What we do is take that circuit and restore the connection to the brain."
She said it was a "really novel" and "refreshingly resourceful" way of restoring movement. However, she warned this would never restore normal function. "That isn't going to happen," she said.
The surgery is not an overnight miracle. It takes intensive training to regain control of the hand. Nerves that used to bend the elbow are now making pinching movements.
After eight months, he was able to move his thumb, index and middle fingers. He can now feed himself and has "rudimentary writing".
With more physiotherapy, doctors expect his movement will continue to improve.
Dr Mark Bacon, the director of research at the charity Spinal Research, told the BBC: "One of the issues with techniques such as this is the permanence of the outcome - once done it is hard to reverse.
"There is an inevitable sacrifice of some healthy function above the injury in order to provide more useful function below.
"This may be entirely acceptable when we are ultimately talking about providing function that leads to a greater quality of life.
"For the limited number of patients that may benefit from this technique this may be seen as a small price to pay."
The technique would work only for patients that have very specific injuries to the spinal cord at the bottom of the neck. If the injury was any higher then there would be no nerve function in the arms to harness. If it was any lower then patients should still be able to move their hands.


More than 3,000 patients have been sent a letter of apology by the NHS after a computer memory stick containing their personal information was found by a member of the public.

East Sussex NHS Trust said the stick had been left by a member of staff near a trust building.
The information was not password protected.
The trust said it took data security "extremely seriously" and the loss was an "isolated incident".
Simon Keen, who was one of the patients whose data was on the stick, said: "They could have credit cards made, things could be purchased online - organised criminals pay a lot for this information.
"Yet the NHS is putting it on a computer stick and a member of the public is finding it in the street
"I find that really disturbing."
The chief executive of East Sussex Healthcare NHS Trust, Darren Grayson, said the data stick belonged to a member of staff and was not compliant with trust policy.
"It was an isolated incident and the trust takes the security of patient's personal information extremely seriously," he said.


A selection of photos from the final week of campaigning in Nigeria's election.

A comedian dances at a rally of Nigeria's ruling People's Democratic Party (PDP) in the south-western town of Akure ahead of Saturday's presidential and parliamentary poll.  Analysts say the vote is evenly split, with the result too close to call but the PDP is confident that it will win its first once more.
The opposition All Progressive Congress (APC) is also confident of victory. Here, its supporters wave brooms in northern Bauchi city to signal their determination to sweep to power in Africa's most populous state.
This man wears a mask of President Goodluck Jonathan in the commercial capital, Lagos. He is facing a strong challenge from Muhammadu Buhari, a former military ruler who hopes to return to office as an elected president.
Here, a vendor sells sunglasses at his roadside shop in front of posters of the bespectacled Gen Buhari. He is campaigning under the slogan "change".
Gen Buhari and Mr Jonathan have also been courting traditional rulers, who wield influence over voters. These traditional rulers met the president in Owerri in south-east Nigeria, in the final days of campaigning....
The election has been marred by militant Islamist group Boko Haram's six-year insurgency in the north-east. Here, a woman lies in a makeshift hospital in Maiduguri, the former headquarters of Boko Haram...
The militants have been in retreat since regional forces launched an offensive earlier this year to recapture territory. In this photo, a Nigerian soldier is seen entering a building in Bama town. The building was used by the militants as a prison, before they the fled.
This woman in the capital, Abuja, buys meat at a market. Some Nigerians have been stocking up on food, amid fears that the election could trigger violence.
Meanwhile in Lagos, a man is seen drawing water from a well while a woman washes dishes. Despite the country being Africa's main oil producer, most Nigerians are poor, hoping that the party they vote for will improve their lives.


Alison Leonard has criticised UK Athletics after she was not selected to compete in the 800m at the Olympic Games in Rio.

The decision was taken despite the 26-year-old running the Olympic qualifying time on six occasions and coming third in the trials.
"I accept I will never win an Olympic medal," she wrote on eightlane.org.
UK Athletics said they do not respond to individual selection policy when asked for a response.
Shelayna Oskan-Clarke and Lynsey Sharp were the only 800m runners named in the Team GB squad for Rio, with a third place left unfilled.
"To leave a space empty on an Olympic team when several athletes have the qualifying standard is an insult to the effort that athletes put in to the sport," said Leonard.
"It tells us that the early mornings, icy runs, hill reps, missed parties and every other thing we do, are not worth anything if you're not going to make the Olympic final.
"For me to be told that the space will be left empty despite being third in the trial... makes me feel I am mediocre."
The Chorley athlete clocked 2 mins 2.45 secs behind Oskan-Clarke and Sharp at the British Championships last month, but said the selection decision had left her feeling like an "also-ran".
She added: "I am an extra on the stage of athletics and no one cares that I practised my lines over and over - I am not needed and my part has been cut."


Nasa has sent a DNA sequencer to the International Space Station in an effort to help astronauts monitor their own health.

A SpaceX cargo ship sent the sequencer into orbit on Monday, along with other items for the crew.
It was developed by the UK-based company Oxford Nanopore Technologies.
The device is designed to show whether DNA sequencing is possible in microgravity.
Nasa hopes DNA sequencers could enable the environmental monitoring of microbes to identify potential causes of illness and understand the health of astronauts.
Last year, Nasa microbiologist Dr Sarah Castro said of the project: "Currently aboard the space station there is not a real-time method for identifying microbes, diagnosing infectious disease, and collecting any form of genomic and genetic data concerning crew health.
"Meeting these needs relies on returning samples from space to Earth and subsequent ground-based analysis, which takes time."
The sequencer, which is just 9.5cm long and weighs 120g, is tiny compared to the microwave-sized devices used on Earth.


When Warren Buffett makes a move, others are sure to follow.

As the world's most successful investor - his company Berkshire Hathaway is worth $200bn (£128bn) -  the decisions he makes have big ramifications for companies, industries and sometimes, for countries.
This week, it has been Australia's turn.
Mr Buffett's A$500m ($386m, £247m) investment in one of this country's biggest insurers, Insurance Australia Group (IAG), has spurred speculation about other companies he might invest in.
Mr Buffett does not like taking risks, a senior analyst at investment research firm Morningstar, David Ellis, told the BBC.
"He wants a reliable return, and that's what the Australian market gives him. It is very mature and well run," explains Mr Ellis about why the American investor from Omaha has invested in IAG.
In a video statement he made overnight, Mr Buffett told his faithful following: "I'm 84 and this is my first investment in an Australian company."
"I've been very derelict but it has been worth waiting for," he said.
The recent decline of the Australian dollar may have prompted his move.
Six months ago, Mr Buffett would have exchanged one Australian dollar for one US dollar. Now he gets a A$1.30-bang for his US buck.
"The expectations are for the [Australian] dollar to keep on falling, and that would mean he can increase his holding [in IAG]," says Mr Ellis.
The current investment in IAG will give the American investor a substantial cash flow which he said he would re-invest in Australia.
His has his eye on Australia's big banks.
"I would say there is a good chance that five years from now, we will have bought one or more positions in Australian banks," he told Fairfax Media in a telephone interview.
Berkshire Hathaway has a track record of buying cheaply and making significant profits, but Australian banks are not cheap.
"If Warren Buffett is interested in banks, there are only four major banks in Australia, and they are relatively expensive compared to others worldwide, for a good reason," says Mr Ellis.
"They are very profitable, they hold strong market positions, and are backed up by reliable regulation."
Julian Lorkin is a Sydney-based business writer.


Weather conditions are too dangerous for searches to continue for a missing man off Jersey, police say.

Roy Deeming's yacht, Desiree, was found drifting off La Corbiere, off the south-west coast, on Christmas Day, prompting a major search operation.
He has been missing from his home in St Brelade since late on Christmas Eve.
Searches were called off on Friday and were not restarting on Saturday, Jersey Police said.


Children with severe epilepsy could be helped by a new treatment derived from the cannabis plant.

The medicine does not contain the ingredient that produces the high associated with recreational cannabis.
The treatment, called Epidiolex, is based on one of the non-psychoactive components of the cannabis plant, CBD.
Early studies in the US have shown treatment with CBD may reduce the frequency and severity of seizures in children with severe forms of epilepsy.
The new trial marks the first time the treatment has been tested in the UK.
Patients are being enrolled for a trial of the treatment at Edinburgh University's Muir Maxwell Epilepsy Centre, based at the Royal Hospital for Sick Children in Edinburgh, and Great Ormond Street Hospital.
The Royal Hospital for Sick Children in Glasgow and Alder Hey Children's Hospital in Liverpool are also driving the study.
There are further centres in the US, France and Poland.
Their initial focus will be on children with Dravet Syndrome, a rare but serious type of epilepsy that is difficult to treat. Some children will receive the treatment while others will receive a placebo.
In a further phase, researchers will also study the effect on children with Lennox-Gastaut Syndrome.
Only children whose seizures cannot be controlled with existing medications will take part in the trial.
Dravet Syndrome usually takes hold in the first year of life. It causes seizures that are often prolonged, lasting longer than five minutes.
They then develop other seizure types. This has a significant impact on the child's development and can be fatal in some cases.
Epidiolex has been developed by the British biotechnology company GW Pharmaceuticals, which is sponsoring and funding the trial.
Dr Richard Chin, director of the Muir Maxwell Epilepsy Centre, said: "Many children with serious forms of epilepsy do not respond to the medications that we currently have available.
"We need new means of treating these conditions so that we can give back some quality of life to these children and their families."


The BBC's Vikas Pandey spends a day inside India's prestigious Jawaharlal Nehru University (JNU) in Delhi amidst the highly charged debate over the arrest of a student leader on sedition charges.

The area near the main administrative block of JNU is filled with passionate students.
They cheer loudly as a speaker climbs on to a stage.
Slogans like "free Kanhaiya Kumar" and "long live revolution" fill the air as hundreds of visibly agitated students pour into the area.
Those gathered here believe that Mr Kumar, the leader of the university's student union, is innocent of the sedition charges levelled against him, and are shocked by the fact that police entered the university to arrest him on Saturday.
Police have alleged he organised an event commemorating the hanging of 2001 Parliament attacks convict Afzal Guru, where "anti-India slogans" were raised.
The students here passionately defend Mr Kumar when I ask them what actually happened at the event.
"We are not terrorists. We are just students and we also condemn anti-India slogans. Our president had nothing to do with those slogans at the event," a student tells me.
But she refuses to speak on camera.
"I don't want to be seen on camera. I am worried about my safety," she says.
Student activist Shreya Ghosh speaks of the fear prevailing inside the university.
"We have been sleeping in different rooms every night to avoid arrest," she says.
Another student activist Deepshita claims that ideological politics lies at the heart of Mr Kumar's arrest.
"Right-wing students want to increase their foothold in the university and that is why they got him [Mr Kumar] arrested. They feel bolstered because the right-wing BJP party is in power at the centre," she says.
Professor Rajarshi Dasgupta agrees.
It's 3pm and speakers are becoming more ferocious in their attack on Prime Minister Narendra Modi and his BJP party.
Among the speakers is Prof Ajith Kanna.
"If Kanhaiya is anti-national, then I am also anti-national," he tells the cheering crowd.
But he pleads with his students to remain peaceful and not pay attention to rumours.
And rumours are not in short supply, flying across the tension filled campus. Among them are that more than 100 armed right-wing activists have entered the campus.
I meet right-wing student group Akhil Bharatiya Vidyarthi Parishad (ABVP) member Saurabh Kumar a few blocks away from the protest site.
"The law will decide whether he [Kanhaiya Kumar] is guilty or not but we won't tolerate anti-India activities inside this campus," he says.
But not everyone is protesting.
At one of the student housing facilities, I meet PhD students Bibas Sewa and Bijay Thapa.
The two condemn the arrest of Mr Kumar, but agree that the protests have disrupted their studies.
Even at a time when politics has gripped JNU, there are some students who just want to get on with their work.


The Maldivian opposition leader and former president, Mohamed Nasheed, has left for the UK where he is due to have back surgery.

Nasheed's departure was delayed by a day after - according to his party - the government imposed new conditions on his trip.
Nasheed was given a 13-year sentence under anti-terror laws last year.
The Maldivian government said Nasheed had signed an undertaking to return after his treatment.
His brother has agreed to act as guarantor, it said.
The government had only agreed to his trip on Saturday following international pressure.
On Sunday his lawyers said the government was insisting that he nominate a family member to stay in the capital, Male, to guarantee his return.
They called this "blackmail".
However the Maldivian Foreign Minister Dunya Maumoon said it was "standard procedure" for any prisoner travelling abroad for medical treatment to sign a guarantee.
"All prisoners who have travelled abroad have signed such documents as required by Maldivian law," he said.
"This rule has been in existence before Mr Nasheed came to office, and has not been introduced in an ad hoc manner."
One of Nasheed's US lawyers, Jared Genser, tweeted that the jailed politician had spoken to US Secretary of State John Kerry by phone.
Nasheed was flying from Male to Colombo, AFP reported, and from there would fly to the UK.
A former human rights campaigner, Nasheed became the nation's first democratically elected leader in 2008, ending three decades of rule by former strongman Maumoon Abdul Gayoom.
In 2012, he was detained after being accused of ordering the arrest of a judge.
He resigned months later amid an army mutiny and public protests over the judge's fate.
Nasheed alleged that he had been removed by a coup, but this was denied by his vice-president, who replaced him.
The current President Abdulla Yameen was elected in controversial polls in 2013 and is the half-brother of Mr Gayoom.


This wildfire has been raging since Tuesday in the American state of California, putting hundreds of homes in serious danger.

Media playback is unsupported on your device
4 September 2014 Last updated at 08:11 BST
Fire fighters said the blaze spread quickly in Siskiyou County. They're are also battling a number of smaller fires.
Around 250 homes have been evacuated, but up to 750 are at risk.
The blaze has already destroyed 100 square miles of forest, but so far no-one's been hurt and no houses have been damaged.
The fire was sparked by lightning in mid-August.


The first robot rover to land on the Moon in nearly 40 years, China's Jade Rabbit, has begun sending back photos, with shots of its lunar lander.

Jade Rabbit rolled down a ramp lowered by the lander and on to the volcanic plain known as Sinus Iridum at 04:35 Beijing time on Saturday (20:35 GMT).
It moved to a spot a few metres away, its historic short journey recorded by the lander.
On Sunday evening the two machines began photographing each other.
A Chinese flag is clearly visible on the Jade Rabbit as it stands deployed on the Moon's surface.
Ma Xingrui, chief commander of China's lunar programme, declared the mission a "complete success".
The first soft landing on the Moon since 1976 is the latest step in China's ambitious space programme, says BBC science reporter Paul Rincon.
The lander will operate there for a year, while the rover is expected to work for some three months.
The Chang'e-3 mission landed some 12 days after being launched atop a Chinese-developed Long March 3B rocket from Xichang in the country's south.
The official Xinhua news service reported that the lander began its descent on Saturday just after 1300 GMT, touching down in Sinus Iridum (the Bay of Rainbows) 11 minutes later.
"I was lucky enough to see a prototype rover in Shanghai a few years ago - it's a wonderful technological achievement to have landed," Prof Andrew Coates, from UCL's Mullard Space Science Laboratory, told BBC News.
Chang'e-3 is the third unmanned rover mission to touch down on the lunar surface, and the first to go there in more than 40 years. The last was an 840kg (1,900lb) Soviet vehicle known as Lunokhod-2, which was kept warm by polonium-210.
But the six-wheeled Chinese vehicle carries a more sophisticated payload, including ground-penetrating radar which will gather measurements of the lunar soil and crust.
On the lander:
On the rover:
True value of Jade Rabbit
The 120kg (260lb) Jade Rabbit rover can reportedly climb slopes of up to 30 degrees and travel at 200m (660ft) per hour.
Its name - chosen in an online poll of 3.4 million voters - derives from an ancient Chinese myth about a rabbit living on the moon as the pet of the lunar goddess Chang'e.
The rover and lander are powered by solar panels but some sources suggest they also carry radioisotope heating units (RHUs), containing plutonium-238 to keep them warm during the cold lunar night.
According to Chinese space scientists, the mission is designed to test new technologies, gather scientific data and build intellectual expertise. It will also scout valuable mineral resources that could one day be mined.
"China's lunar programme is an important component of mankind's activities to explore [the] peaceful use of space," Sun Huixian, a space engineer with the Chinese lunar programme, told Xinhua.
After this, a mission to bring samples of lunar soil back to Earth is planned for 2017. And this may set the stage for further robotic missions, and - perhaps - a crewed lunar mission in the 2020s.
Dean Cheng, a senior research fellow at the Heritage Foundation, a conservative think-tank in Washington DC, said China's space programme was a good fit with China's concept of "comprehensive national power". This might be described as a measure of a state's all-round capabilities.
But he said that China did not see itself as being in a "space race" with anyone else. "I'm comparing it specifically to how the US and the Soviets were behaving in the late 1950s and 1960s when you had space launches almost every month.
"Look at how often the Chinese do manned missions - it's almost every other year."
Instead, the country is methodically and patiently building up the key elements needed for an advanced space programme - from launchers to manned missions in Earth orbit to unmanned planetary craft - and it is investing heavily.
Mr Cheng added: "China is saying: 'We are doing something that only two other countries have done before - the US and the Soviet Union."
The landing site of Sinus Iridum is a flat volcanic plain, part of a larger feature known as Mare Imbrium that forms the right eye of the "Man in the Moon".


News that Abdelhamid Abaaoud, believed to be the ringleader of the Paris attacks, had died during a French police raid came as a welcome win for investigators.

At the same time though, they realise the Belgian jihadist's whereabouts was just one strand in a complex and evolving investigation being pursued under the enormous pressure created by the knowledge that their enemy may launch murderous follow-on attacks at any moment.
On Wednesday evening, Paris prosecutor Francois Molins caused consternation when he revealed that "a new cell was neutralised" during the bloody raid of a flat in Saint Denis and that "this team would have been able to carry on other attacks".
Suddenly we realised this was not those who'd escaped Friday's atrocities and that even if Abaaoud was proven to have been there, it was a different cell, heavily armed, that according to some official briefings was about to perpetrate a massacre in the financial district of La Defense.
So we have received a salutary reminder that the journalistic narratives of an "eighth man" or an "escaped mastermind" often get in the way of understanding what is happening in a case like this.
It might be done with integrity (based on official sources leaking information for their own reasons) or it might be no more than a social media echo chamber where some speculation soon becomes a "fact" bemused investigators are then quizzed about.
Their inability to answer some of these questions might be seen as suspicious, whereas in fact it is no more than puzzlement.
The question of how many people the authorities are looking for is in itself a complex one because the number is being added to all the time.
When Mr Molins revealed on Wednesday, for example, that one of Friday's Stade de France attackers and one of the Bataclan theatre killers remained unidentified, it indicated a certainty that once those identities had been established it would generate additional lines of inquiry, raids and quite probably arrests.
There have been so many of these police swoops - several hundred since Friday - that one has to ask whether they are simply taking this opportunity to bring in all sorts of jihadist suspects or whether the list of people they are looking for is much larger than we might suppose.
After all, there are all manner of enablers or supporters who may be questioned for helping jihadist cells or simply complicit by their failure to report certain facts, even if they didn't have a direct role in the attack.
Fanning the flames of uncertainty to achieve maximum effect, the Islamic State (IS) group on Thursday released two videos promising more attacks.
"Christian countries," one speaker warned, "suicide bombers are present in your country in their tens, or rather in their hundreds."
While one might dismiss this as mere propaganda, it's clear there are hundreds of returned jihadists in Western Europe, even if relatively few of them would be prepared to carry out an IS "martyrdom operation".
Returning to the central issue of the 13 November perpetrators and their immediate network, there are reasons for believing it is considerably bigger than the eight men who carried out the attack.
The day after these atrocities, the Iraqi government revealed it had warned France that 25 IS members were on their way from Syria.
Now this may be wrong, not least in that it has not been proven that all of last Friday's attackers were recently arrived from the Middle East, but the discovery of a new, but connected, cell in Saint Denis, and of a car full of guns and explosives in Bavaria (a few days before Paris, but known to have been heading there), provide strong indications that Abaaoud and IS were preparing something more extensive than the eight-man assault that took place.
The news that Abaaoud was indeed one of those who died in the Saint Denis raid of course creates new concerns among intelligence agencies. How could an IS "poster boy" who had appeared in the movement's videos and been on counter-terrorist watch lists across Europe have made his way to Paris undiscovered?
As if this were not already disturbing enough, the authorities have added to the dread. French Prime Minister Manuel Valls saying: "We must not rule out anything," has suggested that IS may be preparing to use chemical or biological weapons in its attacks.
French police have been authorised to carry their firearms at all times, on or off duty.
Reminding the public of these concerns (IS is believed to have used mustard gas chemical agent against opponents in Syria) may be no more than an ugly necessity at this time.
But of course Mr Valls was on Thursday guiding legislation for a three-month extension to France's state of emergency (giving sweeping powers to the authorities) through parliament.
As in the UK, this involves a difficult public debate about intrusion into people's privacy and the suspension of some cherished civil liberties.
Whatever our uncertainties about the investigation, we have learned enough already to understand that it is a widening dragnet that is expanding across Europe. Multiple raids in the Molenbeek district of Brussels have been generated by intelligence coming from Paris, as well as arrests in Germany.
Each intervention is producing new leads, and there are investigative or intelligence teams across Europe now re-examining earlier incidents, mapping individuals that might be connected to the latest discoveries.
It is a huge task and little wonder that the police or intelligence bureaucracies are taking their opportunity to push for bigger budgets and more people.
At some point, all of this will peak and the number of leads or files being closed off by investigators will start to exceed the new ones being pursued.
Many suspects on the outer edges of the conspiracy or simply in the wrong place at the wrong time will be released. There may even be some sense of relief.
But of course that will simply mark the end of the story of the Abaaoud network and the 13 November attacks.
It would be foolish to think that Islamic State will not be trying to reinvent this model all over again with new teams of operatives dispatched to spread death on the  streets of Europe.


Home Secretary Amber Rudd has written to Southern rail calling for "adequate" bus replacement services to be provided in Sussex during the next RMT strike.

In a letter with Bexhill and Battle MP Huw Merriman, the Hastings and Rye MP said constituents would otherwise face higher costs and longer journeys.
They also expressed disappointment about the lack of reinstatement of the emergency timetable across Sussex.
RMT members are due to stage a 48-hour strike on 7 and 8 September.
The rail union is fighting moves to turn conductors on Southern into "on-board supervisors", with drivers taking over responsibility for opening and closing carriage doors.
The union has said it has concerns over safety and job cuts, but the company began imposing the changes earlier in August.
In their letter to the managing director of Southern rail, Charles Horton, the MPs said the industrial action was "regretted in circumstances where the RMT have been offered guarantees on jobs and pay".
Ms Rudd and Mr Merriman said bus replacement services would "minimise the impact" of next week's strike.
"A failure to provide a bus replacement service sends out the signal that constituents closer to the coast are not treated with the same sense of urgency as those closer to London.
"This has already been exacerbated by the news this week that certain services which had been subject to the temporary timetable have been returned to normal in south London and Surrey but not in Sussex," they said.
The MPs went on to point out that their constituents, unlike other parts of the network, would not be able "to enjoy the benefits of the new driver-operated trains", and were suffering "more detriment than others".
Mr Merriman later told BBC South East that he had received "a communication" from Southern saying the timetable to the coastal routes in Sussex would be reinstated "within weeks".
The operator introduced a reduced service in July after months of delays and cancellations, which it blamed on "unprecedented" staff sickness.


Celtic clocked up another dominant performance and victory over Kilmarnock thanks to strikes by James Forrest and Callum McGregor.

The goals came late in both halves, and were created by the vision and touch of Tom Rogic.
Celtic were not at full strength, making six changes to the starting line-up, but still commanded the game.
Kilmarnock were restricted to half chances and unable to disrupt the visitors' accomplished passing.
The victory - Celtic are now 52 domestic games unbeaten - moved the defending champions top of the Scottish Premiership, with the rest of the teams still to play later in the afternoon.
Kilmarnock have now lost all three of their opening league games for the first time since 1995.
Brendan Rodgers' team selection was an expression of Celtic's growing dominance. Only five players remained from the side that began Wednesday's Champions League play-off tie against Astana, and five of the starters at Rugby Park were graduates from the club's youth academy.
The back line, in particular, was testament to Celtic's potential strength in depth, with Kieran Tierney the eldest at 20, alongside Anthony Ralston, 18, Kristoffer Ajer, 19, and Calvin Miller, 19. Elsewhere, Kundai Benyu and McGregor were provided with opportunities to impress and Forrest returned to his makeshift centre-forward role.
Kilmarnock ought to have responded with a sense of purpose against a visiting side that was not at full strength, but the recent 5-0 Scottish League Cup defeat to another Celtic side that contained several fringe players, would also have played on their minds.
The home side conceded territory and possession to Celtic, as they sat off the visitors, stayed behind the ball and attempted to close off their passing angles. Celtic are accustomed to facing that level of containment, though, and they merely passed from side to side with an air of calm governance.
Rodgers talks often about the strategy of his side, and elements of this display were recognisable, such as Miller playing high and wide on the left, Benyu tucking in as another central attacker, and Ralston dropping back when attacks were on the left to leave a solid back three.
Patience rather than urgency shaped Celtic's approach. Scott Brown was central, collecting the ball from his centre-backs, moving it from flank to flank, and also providing a further level of defensive security on the rare occasions when Kilmarnock carried the ball forward.
There were moments for Celtic to endure, as Ralston headed a Stephen O'Donnell cross wide of his own post, but otherwise the visitors could play as if convinced of their eventual breakthrough.
It came when Rogic embarked on one of his ghosting runs into the Kilmarnock penalty area, ambling past tackles before slipping the ball back for Forrest to side-foot into the net.
Brown, with a header wide, and Miller, with a ball that flashed across the face of the goal, also troubled Kilmarnock in the opening half. The left-back might also have added to the advantage after the break after linking slickly with Forrest, but stabbed the ball straight at Kilmarnock goalkeeper Jamie MacDonald.
The Rugby Park manager Lee McCulloch tried to alter the game's dynamic by withdrawing a midfielder in Steven Smith and replacing him with the striker Eamonn Brophy, but Celtic took further control.
Rogic flitted in and out of the game, but his touches always carried the potential to be flourishes of creativity. With a deft shuffle and release of the ball, he sent McGregor into the penalty area late on, and the midfielder calmly steered a shot past the exposed MacDonald.
The goal ended the tie as a contest, but in truth Celtic had never looked in anything other than total control.
Match ends, Kilmarnock 0, Celtic 2.
Second Half ends, Kilmarnock 0, Celtic 2.
Foul by Jordan Jones (Kilmarnock).
Callum McGregor (Celtic) wins a free kick on the left wing.
Corner,  Kilmarnock. Conceded by Anthony Ralston.
Goal!  Kilmarnock 0, Celtic 2. Callum McGregor (Celtic) right footed shot from the centre of the box to the bottom left corner. Assisted by Tomas Rogic.
Foul by Anthony Ralston (Celtic).
Stephen O'Donnell (Kilmarnock) wins a free kick on the left wing.
Attempt blocked. Leigh Griffiths (Celtic) left footed shot from outside the box is blocked.
Rory McKenzie (Kilmarnock) is shown the yellow card for a bad foul.
Stuart Armstrong (Celtic) wins a free kick in the attacking half.
Foul by Rory McKenzie (Kilmarnock).
Foul by Leigh Griffiths (Celtic).
Greg Taylor (Kilmarnock) wins a free kick in the attacking half.
Attempt missed. Kris Boyd (Kilmarnock) right footed shot from outside the box is close, but misses to the left from a direct free kick.
Foul by Tomas Rogic (Celtic).
Adam Frizzell (Kilmarnock) wins a free kick in the attacking half.
Substitution, Celtic. Mikael Lustig replaces Kundai Benyu.
Kundai Benyu (Celtic) wins a free kick in the defensive half.
Foul by Gordon Greer (Kilmarnock).
Attempt missed. Eamonn Brophy (Kilmarnock) right footed shot from the left side of the box misses to the right following a corner.
Corner,  Kilmarnock. Conceded by Calvin Miller.
Substitution, Kilmarnock. Chris Burke replaces Dom Thomas.
Jordan Jones (Kilmarnock) wins a free kick in the defensive half.
Foul by Anthony Ralston (Celtic).
Kristoffer Ajer (Celtic) wins a free kick in the defensive half.
Foul by Eamonn Brophy (Kilmarnock).
Attempt blocked. Nir Bitton (Celtic) right footed shot from the centre of the box is blocked.
Substitution, Celtic. Nir Bitton replaces Kieran Tierney.
Substitution, Celtic. Leigh Griffiths replaces James Forrest.
Corner,  Celtic. Conceded by Stephen O'Donnell.
Attempt saved. Calvin Miller (Celtic) left footed shot from the centre of the box is saved in the bottom right corner.
Attempt saved. Kieran Tierney (Celtic) left footed shot from outside the box is saved in the centre of the goal.
Attempt missed. Calvin Miller (Celtic) right footed shot from the left side of the box is too high.
Corner,  Kilmarnock. Conceded by Kieran Tierney.
Substitution, Kilmarnock. Eamonn Brophy replaces Steven Smith.
Stephen O'Donnell (Kilmarnock) is shown the yellow card for a bad foul.
Stuart Armstrong (Celtic) wins a free kick in the defensive half.
Foul by Stephen O'Donnell (Kilmarnock).
Corner,  Celtic. Conceded by Stephen O'Donnell.


National League South side Whitehawk have abandoned plans to change their name to Brighton City following discussions with supporters.

Chairman John Summers said it had been "impossible" to notify fans before they submitted an application to the Football Association in December.
However, the Hawks have withdrawn their application after Summers spoke to fans during Saturday's defeat at Chelmsford.
"We have listened to fans," Summers told the club website.
Whitehawk, who are based in a suburb in the east of Brighton, are currently ninth in National League South and reached the second round of the FA Cup this season before being knocked out by Dagenham & Redbridge.
Summers said in a statement on the club website that it had previously been "unanimously agreed" that a name change would be required "in order to widen the club's appeal and to embrace the city as a whole".
However, after notifying fans last week that they had submitted an application to change name to the FA, a section of Whitehawk supporters set up an online petition opposing the move.
"We were aware that sentiments may have changed, especially since the raising of the club's profile during the recent cup run," Summers added.
"I made a point of standing on the terraces at Chelmsford to engage with fans and they made it abundantly clear to me that although they understood the need to embrace the city, they dismissed the name Brighton City as their preferred choice.
"It was good to be amongst them and listen to their views and I thank them all for their input.
"They have made it very clear to us that if we are changing our name they prefer other options such as Brighton Whitehawk or Brighton East End."


Voters have been to the polls to elect 122 of the 123 members of Cornwall Council.

Counting starts at 10:00 BST on Friday and the result will be posted on the council's website.
The local authority has been led by a coalition of Liberal Democrats and Independents since 2013.
One contest, in Bodmin St Petroc, has been delayed following the death of Liberal Democrat councillor Steve Rogerson.
The Council of the Isles of Scilly has also been electing 16 councillors.


A teenage mother had her child tax credits stopped after she was wrongly accused of being married to a dead 74-year-old man, the BBC has learned.

Nicola McKenzie is one of hundreds of people complaining they have been incorrectly punished by the US firm Concentrix, used by the government to cut tax credit fraud and overpayment.
She is struggling to feed her son, she told the Victoria Derbyshire programme.
The government said Concentrix would "not be paid" for wrong decisions.
The Treasury has revealed 120 cases since last October where the company did not "fully" meet the performance standards laid out in its contract.
But it is feared the actual number may be higher. The Facebook campaign group Concentrix Mums, which has 5,600 members, says hundreds more people have been affected by errors.
Labour MP Louise Haigh is demanding that Concentrix be stripped of its contract, saying: "They've stopped people's benefits on a quite incredible scale in order to chase profits."
Ms McKenzie, a 19-year-old single mother from West Yorkshire, received a letter from the company saying her child tax credits would be cancelled.
"Basically they were accusing me of being married to a 74-year-old bloke that used to live here way before I did, saying that it's a normal thing for my kind of age," she said.
"When I spoke to the council, they said he was deceased and died on 5 July 2016," Ms McKenzie added. "Then they said 'Well, you still need to get him to make contact with us'. And I said 'Well, heaven doesn't have opening hours, so what do you want me to do about that?'."
Ms McKenzie said she had lost £64 a week, which meant she was finding it difficult to provide nappies and food for her two-year-old son.
In May last year, Concentrix won a £75m contract to try to save the government more than £1bn in incorrect or fraudulent tax credit payments.
Tax credits - the Child Tax Credit and the Working Tax Credit - are government payments made to households on low incomes.
Concentrix's government contract is based on a payment-by-results model, with the "maximisation of revenue flows" as one of its key requirements - meaning it makes more money if it cuts more payments.
Sharon Scargill, a mother from West Yorkshire, told the Victoria Derbyshire programme her tax credits had been cut after she received a letter from the company saying she was living with another woman of whom she had never heard.
"I've got £20 to last me two weeks, as of yesterday, to feed me and my daughter and for the bills that I'm supposed to pay and the gas and electric," she said.
Ms Haigh, MP for Sheffield Heeley, said some people receiving the letters were confusing them with a phishing scam, as they asked for "very personal details", such as bank statements and information on partners' details. The problems with Concentrix were on a "grand scale", she added.
But an HM Revenue and Customs spokesperson said support was "available for those who need it", adding: "We take great care to make sure that correct tax credit payments are made to prevent customers getting into debt, which they may struggle to repay.
"Payments to Concentrix are based on the quality and accuracy of their work. They will not be paid if the decisions they make about claimants' awards are wrong."
A Concentrix spokesman said: "We recognise that the re-evaluation of individual tax credits claims can be difficult for all concerned.
"We adopt a rigorous process at every stage to ensure we manage this process responsibly, and in full accordance with the protocols and guidance set by HMRC."
The Victoria Derbyshire programme is broadcast on weekdays between 09:00 and 11:00 on BBC Two and the BBC News channel.


Two new therapies for hard to treat advanced kidney cancer could change treatment of the disease, say experts at the European Cancer Congress.

Both drugs increased survival in trials which are also published in the New England Journal of Medicine.
One drug takes the brakes off the immune system while the other stops growth signals in the tumour.
Cancer Research UK said the developments will "greatly expand the arsenal" of available drugs.
Kidney cancer is the eighth most common cancer in the UK and survival rates plummet if it is caught late.
Once the tumour has spread to other parts of the body then only one-in-10 people live for five years after diagnosis.
The first trial, called Checkmate 025, used the immunotherapy drug nivolumab.
It is one of a suite of "checkpoint inhibitors" being developed by pharmaceutical companies that stop cancers turning off the immune system.
They have already been proven effective in skin and lung cancers.
The trial on 821 patients showed average survival was increased from 19.6 months with standard therapy, to 25 months with nivolumab.
Dr James Larkin, a consultant at the Royal Marsden Hospital, told the BBC News website: "It's another big day for immunotherapy for cancer and one of the biggest days for kidney cancer for some time.
"We've known for two to three years that these drugs have efficacy in multiple types of cancer, but it's the randomised control trials that are important."
The second trial, Meteor, used the targeted therapy cabozantinib on a trial of 658 patients.
It doubled survival from 3.8 months to 7.4 months.
Prof Toni Choueiri, from Harvard Medical School, said: "An early evaluation of overall survival from the ongoing Meteor trial has shown a strong trend indicating that survival may be improved in patients receiving cabozantinib compared to standard therapy."
Commenting on the findings, Dr Alan Worsley from Cancer Research UK, said: "Advanced kidney cancer has been hard to treat for far too long and it has been particularly difficult to find drugs that work after first-line treatment has failed.
"The drugs tested in these two trials both appear to work better than everolimus - one of the options available if the first treatment fails - and with fewer side-effects.
"Cabozantinib, a targeted therapy, and nivolumab, an immunotherapy, fight cancer in very different ways, so making either available for use in the clinic will greatly expand the arsenal for clinicians to treat kidney cancer patients."
Prof Peter Naredi, the scientific co-chair of the Congress, said he was "excited over the advances" and that the results "most likely will be practice-changing".


A legend of Celtic mythology will be keeping watch over houses in Londonderry this summer, thanks to a new tree sculpture.

Due to be chopped down, the Beech tree at the Belmont estate has instead been transformed into a piece of public art.
The dead timber is being shaped by a local sculptor and wood carver, in cooperation with the Housing Executive.
The sculpture will tell the story of Bran Mac Feabhail, a Celtic demigod, who features in Irish mythology.
Speaking to BBC Radio Foyle, sculptor Jim Hughes said: "I've been doing some stuff for the housing executive social investment fund and they gave me funding for tools and we ended up coming up with the idea of doing Bran Mac Feabhal."
He explained: "Bran means crow, Mac means 'son of' and an Feabhal is the (river) Foyle. So I'm doing an image of Bran on the top and then we'll fit in an image of an Feabhal (the Foyle) as well.
"Where did the tribe come from? We're trying to tell that story so we're almost doing it like a family tree on a tree."
The innovative project began when a local resident got in touch with the Northern Ireland Housing Executive (NIHE) about a dying tree.
"We had a call about six weeks ago from a private tenant down here at Belmont who was a bit worried about a tree," NIHE's Stephen Proctor said.
"We looked at the tree, there was severe retrenching in the crown of the tree, we assessed the risk and decided it was necessary to remove the tree.
"But we had been working with Jim before and because of the width of this beech tree, we thought we might look into getting a sculpture on to the stem.
"It escalated from that and it's been a good news story all round," he added.
Jim Hughes is part of a two-man team which includes local wood carver Sean Carr.
Mr Carr, who was a commercial diver, began to hone his carving skills after he found a bit of driftwood and decided to turn his pocket knife to it.
"I love woodcarving and I think this is brilliant," he said.
"Instead of taking these trees down, cutting them up for firewood or shredding them, we sculpt them."
The work will be finished in August and it is hoped that the tree, which is already 60 years old, could survive for another half century.


Kim Dotcom, the multimillionaire founder of closed file-sharing site Megaupload, has won back cars, cash and property seized in a January 2012 raid.

Requests from police to extend the length of time the assets could be held were declined by a court in Auckland, New Zealand, where Mr Dotcom lives.
US authorities accuse Mr Dotcom of infringing copyright on a "massive scale".
He has also been sued by several major film and music publishers.
They accuse him of using Megaupload to encourage users to upload copyrighted material illegally - and profiting from the operation.
Mr Dotcom, along with several associates, has denied the charges.
Since the raid, he has launched a new file storage site, Mega, as well as setting up his own political party.
Police raided the mansion of Mr Dotcom and his wife Mona two years ago, taking more than 15 luxury vehicles, including a pink 1959 Cadillac.
Mr Dotcom tweeted: "Breaking News: High Court ruling just now. Mona and I are getting our New Zealand assets back, unless the Crown appeals."
The authorities have 14 days to appeal against the decision.
Mr Dotcom added: "The NZ asset ruling is HUGE. We've just filed a case in Hong Kong against unlawful seizure of #Megaupload. The US case is falling apart!"
An extradition hearing is due to take place in July. If extradited and brought to trial, Mr Dotcom faces 20 years in prison.


The leader of the Pacific nation of Niue has mounted a robust defence of stamps marking Britain's royal wedding which have a perforated line that splits the happy couple.

Premier Toke Talagi admitted the stamps were "unusual" but said they showed Niue was celebrating the marriage.
The stamps are sold as a pair for NZ$5.80 ($4.53, Â£2.79), but can be torn down the middle.
Prince William's stamp is NZ$3.40 and Kate Middleton's is cheaper at NZ$2.40.
The stamps were designed and printed by New Zealand Post.
Niue, although self-governing, is in free association with New Zealand. All Niueans are New Zealand citizens and Queen Elizabeth II is their head of state.
Ivor Masters, the general manager of stamps and coins at New Zealand Post, said the stamps were collectors' items and so unlikely to be separated for postage.
And Mr Talagi said he saw nothing wrong with them.
"People indicated the stamps... meant the couple will separate in future. I don't know why they would interpret it that way," he told AFP news agency.
"I don't think it means that. I think it means we're very happy celebrating the royal marriage."
The stamps might even draw tourists to Niue, he added, because people would want to see where they were from.
The royal wedding takes place in London on 29 April.


The Thai parent company of a threatened Teesside steel plant has been criticised for its "deafening silence".

Teesside steel firm SSI has suspended production at its Redcar blast furnace, blaming rising costs and a slump in demand.
Unite union branch secretary Kevin Cook said there was only enough coal to keep the coke ovens burning until Friday.
SSI has declined requests for comment and unions said they had not been told what a "pause" in production meant.
Community union chairman Paul Warren said it was "torture" for the almost 2,000 workers.
"We're getting no answers and no information," he said.
"We really, really need SSI Thailand to come forward to let us know what's going on."
He said that although the unions' relationship with the firm's UK board was good, from Thailand the "silence is just deafening".
Contractors have been seen withdrawing what they own from the site.
Redcar Labour MP Anna Turley said there was "serious risk that the site is in jeopardy".
"We've been desperately looking for an injection of cash, some support from government but, more than that, just the commitment from SSI in Thailand to say that they believe in the future of this site," she said.
Workers were looking to the government and its support for a Northern Powerhouse to see "if they put their money where their mouth is", she said.
Northern Powerhouse minister James Wharton said the situation was "incredibly serious" and the government had an "open dialogue" with SSI in the UK and Thailand.
"I don't want to sugar coat what is a very difficult period and difficult decisions that could still be to come," he said.
The Redcar plant can produce up to 400 slabs of steel a day, each weighing up to 33 tonnes.
About Â£1bn was invested in reopening the blast furnace.


England's schools face "disruption" after ministers changed their minds about which GCSEs will count in school league tables, says the heads' union ASCL.

Ministers are phasing in new, toughened GCSEs and originally said the old ones would count in league tables during the transition to the new exams.
Now old GCSEs will count for students but not for school accountability.
The government said the move meant consistency between exams.
Ministers have pushed through a range of changes to GCSE and A-level exams which require complete new syllabuses and examinations to be drawn up, approved and accredited.
English and maths GCSEs are being introduced this year, with other subjects to reach schools for teaching in 2016.
Many schools, however, run three-year GCSE programmes, which start in Year 9, with some exams being taken in Year 10 and some in Year 11.
This enables students to spread exams over two years, rather than having to take a large number of exams in one, and creates space in the timetable to focus on key subjects such as English and maths in Year 11.
Students starting these three-year courses in September cannot take the reformed GCSEs in Year 10 because they are not yet available. So, schools had planned that GCSEs taken in Year 10 would be in the unreformed older courses.
ASCL president Peter Kent said: "School leaders are now left with an invidious dilemma.
"They must either change decisions already made with students and parents, and rip up existing timetables, to make sure everybody takes the new reformed GCSEs, or press on with the plans they already had in place and accept that their performance tables will suffer because they will include unreformed GCSEs.
"This has plunged schools, students and parents into disruption and uncertainty."
Dr Kent said: "It is absolute nonsense. These are perfectly valid qualifications which reflect the achievements of students, but which schools will not be allowed to include in their figures.
"One subject which is particularly affected is religious education because this is a GCSE which is sat in many schools in Year 10."
Mr Kent, who is also head teacher of Lawrence Sheriff School in Rugby, said: "In my school we have decided to take the hit on performance tables, but there is no right answer. It is a case of damned-if-you-do and damned-if-you-don't.
"ASCL is very disappointed that despite clearly showing to ministers and officials that this decision is profoundly unfair, they have refused to budge. We have asked them to be reasonable, but our appeal has fallen on deaf ears."
A Department for Education spokesman said: "Our new world-class GCSEs are raising the bar so more young people have access to the world-class education they deserve, equipping them with the skills they need to get on and succeed in life.
"We want pupils to be taught these new gold standard GCSE courses as soon as possible. "


The FTSE 100 slumped 3.5% as investors fretted over global growth prospects and falling oil prices.

The UK's benchmark index closed down 203.2 points at 5673.58, and has now entered a "bear market" having fallen 20% from its record high in April.
Indexes across Europe also tumbled, with Germany's Dax down 2.8% and the Cac-40 in Paris dropping 3.5%.
Shares in Shell were down about 7% after it said that annual profits would be slightly below City expectations.
Oil shares were also hit by the continued fall in crude prices. Brent crude fell 4% $1.16 to $27.60, while US crude dropped more than 5% to $27.01.
Crude oil prices have been falling since 2014 but despite that fall, producer countries have maintained output.
On Tuesday, the International Energy Agency warned that oil markets could "drown in oversupply" in 2016.
Mining shares were also hit hard. Glencore shares fell nearly 10% while BHP Billiton fell more than 7%.
BHP Billiton released a production report containing what investors interpreted as gloomy comments about the outlook for commodity prices.
Shares in WH Smith led the FTSE 250 higher, with a 5.8% gain. The company said it expects annual profits to be "slightly ahead" of expectations, due to strong sales over the five-week Christmas period.
On the currency markets the pound was one fifth of a cent higher against the dollar at $1.4178, and one tenth of a euro cent higher against the euro at â‚¬1.2990.


Almost 1,200 people have signed a petition against the closure of a Heinz food factory in Norfolk.

Two hundred jobs are at risk at the Westwick plant in North Walsham, which produces Aunt Bessie's potato products.
Denise Burke, from the North Norfolk Labour Party, which set up the petition, said the closure would have a "substantial impact" on the local area.
A Heinz spokesman said the firm was "actively exploring all options" before a final decision was made.
The proposed closure, announced earlier this month, comes as Heinz's licence to manufacture Aunt Bessie's products comes to an end in April.
Ms Burke said some aspects of the talks with the company had been "positive", including a possible extension to the 45-day consultation period with staff.
But she said it had yet to address a number of concerns or undertake an equality impact assessment, to analyse the effect of the closure on the area.
"If the site closes, it will have a substantial impact on the local economy," said Ms Burke, who believes unemployment in the area could rise by as much as 30%.
"It's incredibly important it stays open because it's not just those who work in the factory that will be affected, it will impact on other jobs such as the farmers who grow the potatoes and the hauliers who deliver them.
"We either need someone else to come in or Heinz to actively look for other contracts."
Heinz said it was "deeply disappointed" at having to make the announcement, adding it would "continue to explore alternatives to avoid or limit the impact of a potential closure".


Up to 90 jobs could go at a Walkers crisp factory under the firm's plans to install new packing equipment.

The jobs at the factory in Lincoln are under threat after parent company PepsiCo announced changes for the Newark Road site.
The factory, which makes Quavers and some of the Walkers Sensation ranges, employs about 250 people.
PepsiCo said it had "looked extensively" at the site and it had been a "tough decision to make".
Spokesman Steve Switzer said: "We are continually reviewing our operations to ensure we remain competitive and take full advantage of future growth opportunities.
"This move will significantly change the way we pack our goods, improve efficiency and ensure Lincoln has the capacity to support the long-term growth of the Walkers business in the UK.
"This has been a tough decision to make and we recognise this is difficult news for some of our employees."
The Lincoln site has been in the city for more than 70 years and was originally the home of Smiths Crisps.
Leader of the city council Ric Metcalfe said job centre staff would be offering support to the affected workers.
He added: "Your heart has to go out to the individual workers and their families in terms of the impact of this when people have already been struggling with pressure on household incomes, and this is just added misery. It is very bad news indeed."
MP for Lincoln Karl McCartney described the news as a "silver lining to a very dark cloud".
He added: "It is upsetting for the individuals affected but the upside is this secures the future of the Walkers site in the centre of Lincoln."


Tyrone will hope that influential Mattie Donnelly avoids censure after TV replays showed him appearing to strike Derry's Chrissy McKaigue in Sunday's Ulster SFC game at Celtic Park.

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McKaigue attempted to block a Donnelly run before the Tyrone man raised his right hand to the Derry player.
Referee Maurice Deegan took no action against the Tyrone centre half-forward.
"It's clear Mattie Donnelly strikes Chrissy McKaigue," said BBC Sport NI's Championship pundit Oisin McConville.
"If that's the case, it's a sending-off."
Donnelly has already been in disciplinary trouble twice this season.
The Trillick man was sent off for striking Cavan's Seanie Johnston in a Dr McKenna Cup in January and was banned retrospectively for one match during the Football League following a post-match altercation with Mayo's Aidan O'Shea in March.
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The incident happened 10 minutes into the second half as Tyrone were on their way to earning a 0-22 to 0-11 victory.
"It's something that the disciplinary committee may have a look at," added McConville.
"It wasn't dealt with on the day so they are able to have another look at it.
"Us looking at the pictures, can you conclusively say it was a punch in the mouth? It's a difficult one.
"I can imagine if Tyrone went up to defend that, they would be able to get (Donnelly) off on this occasion.
"(But) From what we saw, it was a clear enough strike. Certainly at that stage, Derry were were well in the game and it (a Donnelly red card) would have made a huge amount of difference to that match."


A female osprey has refused to give up on her clutch of three eggs even after her nest was smothered in snow.

EJ, who is 20-years-old this year, has been visiting RSPB Scotland's Loch Garten reserve near Grantown on Spey in the Cairngorms for 15 years.
The bird of prey migrates from west Africa.
Staff at the reserve said EJ had experienced wintry conditions before and the snow could even act as an insulator.
However, the staff added that they could not remember EJ, whose large nest is in a tree, encountering so much snow during her visits to the loch.
The Cairngorms have experienced heavy snow falls this week.
Loch Garten reserve's Julie Quirie said: "I've worked at the centre for 10 years and I just can't remember the snow ever being so bad.
"Poor old EJ does look pretty miserable in her snow doughnut, as we like to call it, but this is her 15th season here at Loch Garten and she's well used to the worst of Scottish spring weather.
"And it's really not as bad as it seems - snow is a good insulator, so as long as this snow snap doesn't persist, EJ and her eggs should be fine.
"It still looks really uncomfortable to us though."
EJ and her mate Odin are the most successful breeding pair at the Loch Garten site.
Over previous seasons 17 of their chicks have fledged.
Ospreys migrate from west Africa to Scotland to breed and can be seen hunting for fish from rivers and lochs.
Ms Quirie said: "All being well, EJ and Odin should be proud parents by the middle of May, when hopefully the skies will be blue and the temperature rising."


Two people have been charged after a man's body was found in a hotel in Woking, Surrey Police said.

Khalilur Rahman died as a result of a head injury, a post-mortem examination has found.
His body was found in the Woking Hotel in Chertsey Road on Wednesday.
Jermaine Lee, 31, of Gorse Road, Croydon has been charged with murder. Lisa Jane Warner, 34, of no fixed address has been charged with assisting an offender.
They have both been remanded in custody and are due to appear at North Surrey Magistrates' court on Monday.


A street in Germany is being renamed in honour of a family persecuted by the Nazis after a Welsh book brought their story to light.

The Bosse family's story was told by Heini Gruffudd, the Welsh language winner of Wales Book of the Year 2013.
His mother, Kate Bosse-Griffiths, was of Jewish decent and fled to Swansea during the reign of the Third Reich.
The street in Wittenberg will be called Bossestrasse following Friday's ceremony.
Mr Gruffudd's grandfather Paul Bosse was a prominent doctor in the town, but the family were targeted by Adolf Hitler's regime because his wife, Kaethe, had Jewish family.
Mr Bosse lost his job and Mrs Bosse died at RavensbrÃ¼ck concentration camp on 16 December 1944.
The ceremony to rename the street falls on the same date and 19 family members from three generations will be there.
Mr Gruffudd said: "The renaming is way of commemorating them, and it's also a small recognition of how they suffered under the Nazi regime."
Prof Dr Hans-Jurgen Grabbe of Wittenberg has been the main force behind efforts to rename the street.
He read an English translation of the book, Yr Erlid (The Persecution), and got in touch with Mr Gruffudd - and is now working on a German-language adaptation, due out in early 2017.
"I became not only interested, but moved by the story," Prof Grabbe told BBC's Cymru Fyw.
"I hit upon the idea of renaming the street - it's to remember not only the doctor [Paul Bosse] but also his wife."
According to Prof Grabbe, the vote to rename the street in the town council was "almost unanimous".
"It's a gesture of atonement," he said. "But the gesture of reconciliation from Heini and his relatives is also important."


The hunt for a cancer cure should be treated with as much urgency as the Ebola outbreak, says US Vice-President Joe Biden.

He said he had dreamed of being the president that cured cancer and believed it was possible.
He is now leading the US "cancer moonshot" programme to cure cancer.
He told scientists their success could "literally change the world" but criticised the barriers to getting on clinical trials.
In a speech to American Society of Clinical Oncology annual meeting he said: "[When] we were worried about Ebola we were able to aggregate tens of millions of dollars and the entire US military because the World Health Organization couldn't handle it.
"That's the kind of urgency we need with regard to cancer."
In January, President Barack Obama announced the $1bn (Â£710m) "moonshot" and that Joe Biden would lead it.
The vice-president said: "If I could have done anything I would have wanted to be the president that ended cancer as we know it because I believe it is now possible."
It is a personal mission for Mr Biden, who lost his son Beau to brain cancer at the age of 46 last year.
He told the meeting of 30,000 of the world's leading cancer scientists and doctors that his son had been able to take part in pioneering clinical trials.
But "what about the 96% of people" who miss out, he asked, and called for new measures to help the poorest patients take part.
"Nobody should have to forgo a critical clinical trial because they cannot afford the gas to get there or a baby sitter at home," he said.
His speech referenced many of the major themes that have emerged at the meeting in Chicago including the transformative power of immunotherapy and the unparalleled understanding of the genetics of cancer.
"[They are] offering profound promise that wasn't there five years ago," he said.
But Mr Biden said the progress could be much faster if only scientists worked closely together.
He said: "Imagine if we all worked together... shared the data behind breakthroughs so that the field as a whole can move forward faster and avoid unnecessary redundancy.
"The whole world is looking to you, your success can literally change the world. We need you now more than we ever have."
Dr Deborah Mayer, one of the expert advisers to the moonshot, said there had been terrific progress in cancer science and the field was now at a "tipping point".
Huge breakthroughs in harnessing the power of the immune system to attack tumours or in tailoring drugs to the weak spot in individual patients' tumours are already helping patients.
In the US, five-year cancer survival has increased from 30% in 1950 to 48% in 1975 and 68% in 2010.
Dr Mayer told the BBC News website that the moonshot would act as a catalyst to bring breakthroughs to patients faster.
"Hopefully it will close that 17-year gap between what we know and what we do.
"The benefit to patients of this moonshot is we will move forward to enhanced treatments, we're going to find cancers earlier when they're more curable and we're going to figure out how to prevent them."
The American Society of Clinical Oncology has recommended four key areas the moonshot needs to deal with.
The organisation's president Dr Julie Vose said: "The Moonshot Initiative can be a vehicle for major new progress against cancer."
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A man found dead in his flat had head injuries, police have said.

The body, found in Tipton, West Midlands, on Wednesday, has not been formally identified but is thought to be that of 24-year-old Indiga Daniel Wallace, known as Danny or Indy.
His death was treated as "unexplained" at first but, following a post-mortem examination, is being treated as murder.
Police said they were trying to put together his last movements.
Anyone with information should contact police. Mr Wallace was found at his home in Fisher Street in the town.
For more updates on this and other stories Birmingham and Black Country


The Commons has rejected calls for Parliament's role in a proposed new system for recalling misbehaving MPs to be substantially reduced.

Conservative MP Zac Goldsmith tabled a proposal which would have excluded Parliament's standards committee from any role in determining whether errant MPs should face re-election.
But MPs rejected the idea by 340 to 166 following a free vote in the Commons.
Introducing recall powers was a key part of the 2010 coalition agreement.
The government-sponsored Recall of MPs bill is being scrutinised in Parliament, after it passed its second reading last week.
The bill proposes that an MP would face a by-election if 10% of constituents sign a petition after the MP is found guilty of "serious wrongdoing".
The bill has been referred to the Committee of the whole House, meaning its individual clauses can be examined by any MP in the House of Commons chamber, rather than the usual committee of between 16 and 50 MPs.
Under the government-backed plans there would be a by-election if 10% of constituents sign a petition after the sitting MP is either sentenced to more than 12 months in jail, or banned from the Commons for more than 21 days.
Mr Goldsmith, MP for Richmond Park and North Kingston, has created a rival plan in which MPs are to face a recall referendum if 5% of voters in a constituency sign a "notice of intent to recall" and 20% then sign a "recall petition".
Only if all three stages are passed would there be a by-election called.
Mr Goldsmith said his amendment would give more power to the public to force a vote but it was overwhelmingly defeated in a Commons vote on Monday.
Speaking during the four-hour debate, Mr Goldsmith urged MPs to put aside concerns about the technicalities and cost of recall elections and focus of the principles involved.
"What is at stake now is a matter of principle - do we trust our voters to hold us to account or not?
"The public today is better informed, better educated, less deferential than at any time in our history. Recall is not radical - it is a mere nod towards those changes."
Constitution Minister Sam Gyimah pledged instead to look at a new Liberal Democrat proposal to create an election court process as an additional "trigger" process outside of Parliament but alongside the internal process.
"If we are going to have a recall system, we should have a recall system the public can trust, that the public understand, and that the public know when they engage in it, it will actually end in a MP being booted out of this House if need be," he said.
The body which hands down bans to MPs is known as the Commons Standards Committee, and is itself mostly made up of MPs. There is some concern that the current bill leaves too much power in the hands of parliamentarians rather than the public.
Although Labour back the bill, shadow minister Stephen Twigg said his party would seek to strengthen it.
He said: "We need a system that improves accountability and one that gives more power to the public to hold their representatives to account between elections. That, I think, is a matter of fairness."
Mr Goldsmith will have the opportunity to propose more amendments to the Bill at the next stage of the legislative process, report stage.


CCTV footage of an armed robber brandishing a sword at women working in a County Antrim filling station has been released by police.

Two masked men robbed the store at Largy Road, Ahoghill, on 26 November.
Police said they had released the video of the young women's "extremely frightening ordeal" in an attempt to identify the men.
A detective said that while the men's faces were covered, "their height and shape might be familiar to someone".
"Also, their gait, stance and movements may provide a further clue as to their identity," said DCI Catherine Magee.
She said anyone who had information but was reluctant to contact police should watch the footage.
"These criminals were quite prepared to use violence and highly threatening behaviour to get their hands on till money," she said.
"It is important that we keep people safe by bringing them before the courts."


Two boys aged 13 and 14 were killed when a car they were travelling in left the road and hit a tree "at speed" in Leeds.

The boys were in a vehicle on Saturday afternoon with two other 14-year-olds who suffered life-threatening injuries.
They were being driven by a 21-year-old man who police think picked up the boys without their parents' permission.
Police said the car had overtaken another vehicle before crashing on Gelderd Road in Morley at 13:50 GMT.
West Yorkshire Police said the two injured 14-year-old boys remained in hospital in a critical condition on Sunday morning.
Supt Sam Millar, of Leeds Police, said: "This is a terrible tragedy for two young boys who had their whole lives ahead of them.
"Specially trained officers are currently investigating the exact circumstances of the incident and helping the families in what is a terrible time for them.
"The two boys were from the local area and attended Bruntcliffe School and the Morley Academy and we are also supporting those schools at this difficult time."
West Yorkshire Police have appealed for any witnesses to how the car, a blue Peugeot, was being driven before the incident.
Supt Millar added: "It is thought that a 21-year-old man picked the four boys up from their homes without the permission of their parents.
"A blue coloured Peugeot was being driven along the road in the direction of Gildersome when the car left the road and collided with a tree.
"No other vehicles are believed to have been involved in the incident although it is thought the car overtook another vehicle at speed during the incident.
"The driver also remains in hospital in a serious but stable condition. Officers are appealing for anyone who saw the movements of the vehicle that day to come forward, especially at the Tesco Express store in Churwell."
Any witnesses are asked to contact the Major Collision and Inquiry Team on 101.
Supt Millar said: "One of the passengers, a 13-year-old boy was killed instantly and one of the other boys in the vehicle subsequently died in hospital last night.
"Two other boys travelling in that vehicle remain critically ill in hospital.
"This is an absolute tragedy for these boys, their families and for the community and for our local community schools.
"What I would like to highlight was that several members of the public who stepped in and helped at the scene of this accident.
"As you can imagine it must have been one of the most horrendous scenes to come across and yet members of the public gave immediate support and first aid to these boys.
"Their actions yesterday are absolutely commendable."


A million more people visited Tate Modern in 2016 compared with the previous year.

More than 5.8 million people went to the London gallery in 2016, an increase of 24%, making it the third most visited attraction in the UK.
No doubt visitors were drawn to Tate Modern's new 10-storey extension, the Switch House, which opened in June.
The British Museum was the most popular visitor attraction in Britain in 2016 for the 10th year running.
The figures come from the Association of Leading Visitor Attractions (ALVA).
The British Museum had more than 6.4 million visits - but that was a 5.9% drop on the previous year.
The top 10 most-visited attractions in the country were all in the capital. Seven of those saw a fall in numbers, including The Natural History Museum and the V&A, which both suffered a drop of 12%.
The overall visitor numbers for London attractions last year were level.
Bernard Donoghue, director of the ALVA, said terrorist attacks in European cities last year had made some people reluctant to visit major cities.
He told the Press Association: "There were some security fears about central London and city centre attractions, both on the part of overseas visitors and also UK domestic families.
"We did see a displacement out of central London for some people going to visitor attractions."
In England, excluding London, there was an increase of 5.9%. Scotland enjoyed the highest increase with 15.6%, while Wales's figures went up by 11%. Northern Ireland's visitor figures increased by 7.6%.
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Canadian Jewish organisations and a human rights lawyer are concerned that a UK blogger accused of Holocaust denial has been allowed into Canada.

Alison Chabloz was a guest at a Calgary event hosted by Blood and Honour, which the Royal Canadian Mounted Police calls a white supremacist group.
Ms Chabloz is facing trial in Britain over an alleged anti-Semitic video posted on YouTube last year.
In the clip, Ms Chabloz appears to mock "fake" Holocaust survivors.
A private prosecution is being brought by the Campaign Against Antisemitism, a UK-based charity, after Britain's Crown Prosecution Service decided not to move ahead with charges.
Ms Chabloz has pleaded not guilty.
B'nai Brith Canada said it reported to Calgary police the Thursday event where Ms Chabloz appeared.
"B'nai Brith is disappointed that Canadian authorities have allowed notorious British Holocaust denier Alison Chabloz to enter this country, especially when the express purpose of her visit is to propagate her anti-Semitic views," the organisation said in a statement.
"Once again, we see Canada disappointingly taking a softer line on hate speech than fellow democracies such as Germany and the UK."
The BBC contacted Ms Chabloz for comment and did not immediately receive a reply.
Earlier this month, Canadian Alfred Schaefer, who was behind a series of anti-Semitic YouTube videos, was charged with criminal incitement in Germany after B'nai Brith Canada reported his activities to officials there.
Calgary police said on Friday they were aware of Ms Chabloz's presence in the city and were monitoring the situation.
Human rights lawyer Richard Warman, who has posted about Ms Chabloz's visit on his website, said he hopes that Canadian immigration officials take a second look at her file and her right to remain in Canada.
"Now that the government has become abundantly aware (of her visit) what will they do?" he asked.
Besides her online videos, Ms Chabloz has faced criticism for performing a quenelle salute  - viewed by many as an anti-Semitic gesture - outside Edinburgh Castle in 2015.
In her blog, she claims she made the gesture in response to a "small group of hard-line Zionists" who she says were targeting her online.
Promoting the Calgary event on its website, Blood and Honour credits Ms Chabloz with popularising in Britain the "anti-Establishment" and "anti-Zionist" gesture.
Blood and Honour promotes neo-Nazi, nationalist and white supremacist ideas and emerged from the 1980s skinhead music scene in England, according to the Southern Poverty Law Centre, an American organisation that tracks hate groups.
On its Canadian website, Blood and Honour describes itself as a movement to "raise awareness of issues concerning our unique and combined European cultures and heritage so that we may preserve and pass on those values to future generations".
The RCMP says the group has branches in Quebec, Ontario and Alberta.
Representatives from Citizenship and Immigration Canada told the BBC they do not comment on specific cases but said all visitors to Canada are carefully screened before being granted entry.
There has been growing concern in the US and the UK over anti-Semitism.
Earlier this month a UK charity reported that incidents of anti-Semitic hate crime were on the rise. In the US, the Department of Homeland Security has offered help to Jewish community centres after they received more than 100 hoax bomb threats in recent weeks.
In Canada a number of recently incidents like vandalism targeting Jewish people have also raised concerns. In 2013, incidents targeting Jewish populations accounting for 56% of religious hate crimes in the country.