David Cameron has returned to Downing Street with the Tories poised to defy polls and win the general election.
The Conservatives made gains in England and Wales and are forecast by the BBC to secure 331 seats in the Commons, giving them a slender majority.
Sources say Ed Miliband is expected to stand down after Labour was all but wiped out by the SNP in Scotland.
Lib Dem leader Nick Clegg has already said he will quit, with his party set to be reduced from 57 to eight MPs.
UKIP leader Nigel Farage is also quitting after he failed to win Thanet South, losing by nearly 2,800 votes to the Conservatives.
In other election developments:
The BBC forecast, with 635 of 650 seats declared, is Conservative 331, Labour 232, the Lib Dems 8, the SNP 56, Plaid Cymru 3, UKIP 1, the Greens 1 and others 19.
The Conservatives are expected to get a 37% share of the national vote, Labour 31%, UKIP 13%, the Lib Dems 8%, the SNP 5%, the Green Party 4% and Plaid Cymru 1%.
Ed Miliband is expected to make a statement about his own future later after what he said was a "difficult and disappointing" night for Labour which saw Ed Balls lose and Jim Murphy and Douglas Alexander losing out to the SNP
Nick Clegg said he would quit as leader after a "crushing" set of losses, which saw Vince Cable, Danny Alexander, David Laws, Simon Hughes and Charles Kennedy among a slew of Lib Dem casualties
George Galloway, who was reported to the police for retweeting an exit poll before voting ended, has lost to Labour in Bradford West
Nigel Farage has quit as UKIP leader after failing to be elected - although he may stand in the ensuing leadership contest. Douglas Carswell retained his Clacton seat
Conservative minister Esther McVey has lost Wirral West to Labour
The Green Party is predicted to get one seat after Caroline Lucas retains the Brighton Pavilion constituency she won in 2010
With 16 seats left to declare, turnout is expected to be 66%, marginally up on 2010
Mr Cameron all but declared victory in a speech after being returned as MP for Witney, in which he set out his intention to press ahead with an in/out referendum on Britain's membership of the European Union and to complete the Conservatives' economic plan.
"My aim remains simple - to govern on the basis of governing for everyone in our United Kingdom," he said.
"I want to bring our country together, our United Kingdom together, not least by implementing as fast as we can the devolution that we rightly promised and came together with other parties to agree both for Wales and for Scotland.
"In short, I want my party, and I hope a government I would like to lead, to reclaim a mantle that we should never have lost - the mantle of One Nation, One United Kingdom. That is how I will govern if I am fortunate enough to form a government in the coming days."
Mr Cameron has returned to Downing Street with his wife Samantha and is expected to hold an audience with the Queen later on Friday.
Chancellor George Osborne said the Conservatives had been "given a mandate to get on with the work we started five years ago" and would follow the "clear instructions" of the British public.
However, Work and Pensions Secretary Iain Duncan Smith acknowledged that governing with a small majority was difficult.
"Whatever else we now do we keep it simple, we keep it focused and we absolutely stick to our manifesto commitments," he told the BBC.
He said the party would deliver an EU referendum as it was a "red line".
Speaking in Doncaster, where he retained his seat, Labour leader Ed Miliband said; "Clearly this has 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 have seen a surge of nationalism overwhelming our party."
He said the next government had a "huge responsibility" and a difficult task to "keep our country together".
After his own defeat, one of the most surprising results of the night, Mr Balls said he had a "sense of sorrow" about his party's disappointing performance but he was "confident that Labour would be back" as a "united and determined" political force.
Mr Cameron looks like he will form a majority Conservative government, without the need for a coalition or the formal support of other parties.
The finishing line needed to form an absolute majority is 326, but because Sinn Fein MPs have not taken up seats and the Speaker does not normally vote, the finishing line has, in practice, been 323. In this election, Sinn Fein kept four seats.
Credit: The BBC