The Conservatives are confident David Cameron will remain PM with early general election results suggesting the party will be close to a majority.
Labour has rejected an exit poll suggesting it would get 239 MPs to the Tories' 316.
The same poll suggested the Lib Dems would lose 47 seats and the SNP would win all but one seat in Scotland.
Results so far suggest the exit poll is accurate but the majority of the 650 seats have yet to declare.If the exit poll is accurate, Mr Cameron will be able to remain in Number 10 as the head of a minority government without the need for a coalition - although he might have to rely on the support of the DUP or the Lib Dems.
Even if Labour leader Ed Miliband was able to persuade the Lib Dems to join the SNP in backing a Labour government, he would not have the necessary numbers to get his legislative programme through Parliament in a Queen's Speech.
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.
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| Election results: Tories confident as Labour falters |
Labour insists Mr Miliband could still be prime minister because the coalition's majority will have disappeared but senior party figures have conceded they are having a disappointing night.Labour is being hammered in Scotland by the SNP, with Nicola Sturgeon's party predicted to take 58 of the 59 seats.
Jim Murphy, leader of the Scottish Labour Party, and shadow foreign secretary Douglas Alexander have both lost their seats to the SNP, which is benefiting from a 27% average swing from Labour.
Conceding defeat, Mr Murphy said it had "proven hard to turn round years of difficulties with the Scottish Labour Party in just five short months".
He congratulated the SNP on the scale of their victory but said he intended to continue as the party' leader in Scotland.
"Scotland needs a strong Labour Party and our fightback starts tomorrow morning," he added.
Labour has failed to make the headway it wanted in the South of England and the Midlands, failing to take its top target seat, Warwickshire North, back from the Conservatives.
Its progress in London has not been as strong as pre-election polls suggested.
There is a recount in Bradford West, where George Galloway is battling Labour to retain the seat he won in a by-election.
Bradford Council has reported Mr Galloway to the police for allegedly breaking election law for tweeting about exit polls before polls closed, the BBC has learned.
The exit poll suggests the Lib Dems will lose 47 seats, with many of their high profile minister and MPs facing defeat, and the party finishing behind the Greens in early results.
Lib Dem election chief Lord Ashdown told the BBC: "If this exit poll is right I will publicly eat my hat."
Analysis by BBC Political Correspondent Chris Mason
Already, Tories are using the language of victory: the Chief Whip Michael Gove told David Dimbleby on BBC One that if the exit poll was right, the Conservatives had "won".
They will make the case that if these numbers are accurate, they are the clear winners, even though they didn't quite make the finish line.
They'll be nervous that whilst they have "clearly won", as Mr Gove puts it, there could still, just, be the potential for an anti-Tory majority - if everyone else clubbed together
David Cameron, arriving at his count in Witney, said it was "early days".
Conservative minster Michael Gove said: "We haven't had an incumbent government increase its majority like this since 1983 and it would be an unprecedented vote of confidence in David Cameron's leadership."
SNP leader Nicola Sturgeon has arrived at the Glasgow count at the Emirates Stadium, where she told reporters she believes it will prove a "very good night for the SNP", adding that she would like to "lock the Tories out of power".
UKIP deputy leader Paul Nuttall expressed doubts over the accuracy of the exit poll, and said the party was "buoyant and confident" that it would win more than two seats.
Giving her comment on the exit poll, Green Party leader Natalie Bennett said: "A doubling of MPs in Parliament would obviously be a huge advance for the Green party."
YouGov poll
Exit pollsters interviewed 22,000 people in 141 polling locations in 133 constituencies throughout Great Britain.
Professor John Curtice of Strathclyde University, who led the exit poll operations, said the results announced so far suggested the exit poll was accurate.
A total of 650 Westminster MPs will be elected, with about 50 million people registered to vote.
There are also more than 9,000 council seats being contested across 279 English local authorities.
Mayors will also be elected in Bedford, Copeland, Leicester, Mansfield, Middlesbrough and Torbay.
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