The result – devastating Labour and the Liberal Democrats, and leaving Scotland a near one-party state under the control of the Scottish National party – probably represents the biggest surprise in a general election since 1945.
Cameron is to head to Buckingham Palace at 12.30pm to meet the Queen and is expected to make a statement on his return to Downing Street.
On a night of heavy losses, Balls, Douglas Alexander, the Labour election chief, and Jim Murphy, the Scottish Labour leader, were among the senior party figures to lose their seats. A roll-call of Liberal Democrat ministers, including Cable, were also defeated, leaving a forlorn Nick Clegg to admit it had been a cruel and punishing night. Clegg narrowly survived in his own Sheffield Hallam seat but will return to Westminster to meet fewer than 10 fellow MPs, down from 57.
Across the Midlands, Scotland and even in London, the swings Labour needed in order to make gains simply failed to appear, and far from a swing to Labour, the results revealed the Conservative party strengthening its vote. The current predictions suggest Cameron will have a small majority, with his party on course to have around 329 seats.
Ed Milband, speaking from Doncaster, effectively conceded defeat as he said he was “deeply sorry” about the result.
He said: “The results are still coming in, but this has clearly been a very disappointing and difficult night for the Labour party. We haven’t made the gains we wanted in England and Wales, and in Scotland we’ve seen a surge of nationalism overwhelm our party. Now, I want to say to all the dedicated and decent colleagues in Scotland who have lost their seats that I am deeply sorry for what has happened.”
It has been argued that a weakened party should not immediately turn in on itself with a divisive leadership contest, something that in 2010 gave room for the Conservatives to shape the political debate.
If the exit poll is borne out in the final Westminster tally, Cameron may be able to govern without the need for the support of the devastated Liberal Democrats or even the Democratic Unionists in Northern Ireland.
However, Cameron is facing the very serious challenge of how to unite the UK after such a comprehensive victory by the Scottish National party in Scotland, where it won all but three seats.
Speaking from his Witney seat in Oxfordshire, Cameron said it had been a “very strong night” for the Conservatives, showing there had been a “positive response to a positive campaign”.
Despite having warned against the threat of Scottish nationalism in Westminster during the campaign, he said now was the time to mend divisions between England and Scotland.
“Above all, I want to bring our country together, our United Kingdom, implementing as fast as we can devolution both for Wales and Scotland,” he said. “I want my party and a government I would like to lead to reclaim the mantle of one nation, one United Kingdom. That is how I will govern if I am fortunate enough to form a government.”
However, George Osborne, the chancellor, told BBC Radio 4’s Today programme that the offer to Scotland would not go beyond the proposals for devolution set out in the Smith Commission.
He said that governing for the whole of the UK would be one of the very big challenges all MPs would face and there was no simple and easy answer.
While Cameron and Osborne were stressing the need to bring the UK together, their political rivals accused them of having put the union at risk.
Balls, having lost his seat in Morley and Outwood, said he now feared for the union, the UK’s future in the European Union and the state of the NHS over the next five years.
The result would appear to be a total vindication of Cameron’s decision to campaign on the threat posed to England by a Miliband government dependent on the support of the Scottish National party.
On a night of carnage for Labour, made worse by the expectations raised by optimistic opinion polls, Miliband appeared to have performed worse than Gordon Brown, losing as many as 20 seats held in 2010.
In a seismic result that brings Scottish independence closer, Labour held on to just one seat in Scotland with the SNP winning all but three of the 59 seats north of the border. Douglas Alexander, the shadow foreign secretary, was the most prominent casualty – losing on a 34% swing to the SNP from Labour, destroyed by a 20-year-old student, Mhairi Black. Alexander said Scotland had chosen to oppose the Conservative government but not to place its trust in Labour.
A second casualty was the Scottish Labour leader, Murphy, who in a dignified concession speech said the troubles facing the Scottish Labour party for many years could not be rectified in five short months. He made clear he would like to continue to lead Scottish Labour.
A shellshocked Labour at first challenged the accuracy of the exit poll, but started to admit as the night wore on that it was broadly accurate. David Blunkett, the former Labour cabinet minister, said: “The exit polls were right, it’s a very bad night for us. We are being swept by the tsunami north of the border.”
But Labour also failed to win key target seats in the Midlands, north-west and Yorkshire, as Ukip voters went to the Conservatives, depriving the party of the swings it needed. In the south, in seats such as Southampton and Swindon, the Lib Dem vote collapsed but did not go to Labour. Miliband’s staff blamed the SNP leader Nicola Sturgeon’s rhetoric vowing to hold sway over a Labour minority government drove English voters back into the arms of the Conservatives. “She ended up in an alliance with the Conservative party,” one said.
Sturgeon was unrepentant, blaming Labour’s failure to win seats in England. She said: “What we’re seeing tonight is Scotland voting to put its trust in the SNP to make Scotland’s voice heard, a clear voice for an end to austerity, better public services and more progressive politics at Westminster. That’s what we now intend to do.” She also repeated her reassurance that the election had not been about Scottish independence.
Alex Salmond said a Scottish lion had roared. But SNP plans to combine with Miliband to lock Cameron out of Downing Street are now redundant.
The initial exit poll projection, greeted with gasps of disbelief, left the Tories 77 seats ahead of Labour, with the combined Labour-SNP tally on 297, still behind the Conservatives. Labour had been hoping the Tories could be pinned back to 280 or 290, enough to block Cameron’s path to securing a majority for a Tory Queen’s speech.
As the night continued, it became apparent that the exit poll may have underestimated the scale of the Tory triumph and the Conservatives were on the brink of an outright majority.
The Liberal Democrats fell as low as 10 seats, beyond their worst nightmare and taking the party back to the Liberals’ status in the 1970s. Clegg,who in 2010 took the party into coalition with the Conservatives, is almost certain to resign at some point on Friday and faces ridicule after he had claimed his party was going to be the surprise success story of the night.
Tim Farron, one of the Lib Dems tipped to succeed Clegg, said he believed fear had won the day, both sides of the border. He insisted Clegg “had done a blinding job”, arguing that Clegg had formed the coalition in the national interest, knowing it was not in the party’s interest.
A cast list of famous Liberal Democrats were defeated including Cable, Danny Alexander, the chief secretary to the Treasury, , former leader Charles Kennedy, education minister David Laws, the energy secretary, Ed Davey, the justice minister, Simon Hughes, the business minister, Jo Swinson, the Home Office minister, Lynn Featherstone, and the veteran Colchester MP Sir Bob Russell. The results suggested the idea that Liberal Democrat MPs would benefit from their incumbency and campaigning track record proved to be a myth.
Ukip was expected to collect two seats, and Nigel Farage, facing a high turnout, was struggling to overcome a tactical vote against him in South Thanet. If he loses, he has said he will quit politics, despite the signs that Ukip was positioning itself as second behind Labour across the north.
An angry Farage said: “I want to congratulate the editors of the Sun and Mail. They’re geniuses. They said the Ukip vote would split the Tory vote. God help us.”
The astonishing nature of the result was partly due to the exit poll being so far out of line with national opinion polls that had showed the two main parties neck and neck, and if anything Labour benefiting from a late surge.
The results were greeted with euphoria at Conservative headquarters. The chief whip, Michael Gove, told the BBC that if the exit poll was accurate it would be the first time a government had increased its majority since 1983 and would represent an unprecedented vote of confidence in Cameron.
He said: “If it is the case that the exit poll is right, then David Cameron has won a very handsome victory in this election. He will have secured both an advance on seats and outperformed the expectation of almost every commentator.”
Gove said the path was set for a stable government, adding that Cameron now had a mandate to press ahead with a referendum on UK membership of the European Union by 2017.
Miliband had led a campaign promising a more equal society, and if he has lost net seats, albeit largely due to the Labour wipeout in Scotland, he will be under pressure to resign. Some of his aides were preparing to argue that he may need to stay if Cameron had only a fragile majority and there was a possibility of a second election. But he will have been decisively rejected and the task of Labour rebuilding in Scotland will be gargantuan.