Prime Minister David Cameron's Conservatives are set to govern Britain for another five years but fall just short of an outright majority, an exit poll showed, a result likely to trigger an in-out EU membership referendum.
The poll forecast the Conservatives would win 316 of 650 seats in the lower house of parliament and the main opposition Labour Party 239.
If accurate, that would be centre-left Labour's worst result in almost three decades and it faced being wiped out in its former Scottish stronghold.
While politicians from all sides expressed caution, early results from 37 seats also suggested Labour was attracting much lower levels of support than it had expected. Pollsters are not ruling out an overall Conservative majority.
"The Conservatives have clearly won this election," if the exit poll is right, said Conservative minister Michael Gove.
Professor Vernon Bogdanor, one of Britain's most foremost constitutional experts, said such an outcome would be "a triumph" for Cameron and that he would be the first premier to gain seats since Margaret Thatcher in 1983.
A Cameron victory would mean Britain would be likely to face a historic in-out European Union referendum within two years and that Scots could soon be pressing for independence again, having lost a secession referendum just last year.
UK election exit polls have a good track record but the large number of parties competing this time has raised the potential for error. It will be well into Friday before final results are announced.
The poll, conducted for Britain's national broadcasters, suggests Cameron will have multiple options to form a government, perhaps with the support of either the Liberal Democrats, his current coalition partners, or Northern Irish unionists or both. He could also try and go it alone.
In practice, controlling 323 of 650 seats in parliament is enough to command a majority so if the Conservatives get 316 seats they would be very close.
The same poll said the Scottish National Party (SNP) would win 58 of Scotland's 59 seats, all but obliterating Labour north of the border.
Opponents fear the SNP is preparing to use an emphatic win to renew its push for an independence referendum even though it lost such a plebiscite only last year.
"We're seeing an electoral tsunami on a gigantic scale," said Alex Salmond, the party's former leader.
"The SNP are going to be impossible to ignore and very difficult to stop," he said, saying such a result would strip Cameron of any legitimacy in Scotland where his Conservative Party would have no lawmakers.
In a body blow, Douglas Alexander, Labour's campaign chief and foreign policy spokesman, lost his seat to a 20-year-old Scottish nationalist student.
The same exit poll suggested the centrist Liberal Democrats, who have governed in coalition with the Conservatives for the past five years, would finish with just 10 seats across the UK.
That would be a disaster for leader and deputy prime minister Nick Clegg.
The UK Independence Party, which wants an immediate British withdrawal from the EU, was on track to get two seats at best amid speculation that Nigel Farage, its leader, would fail to be elected and therefore have to step down.
Before the election, opinion polls had shown the Conservatives and Labour neck-and-neck, leaving that industry facing a potential inquest.
If the main exit poll is accurate, Cameron's position as Conservative leader, which had been looking shaky before the election, would be secure. By exceeding expectations he will
expect to quell dissent within his party.
Conversely, the result would be a crushing defeat for Labour and Ed Miliband, its leader.
Even if the centre-left party got together with the left-leaning Scottish nationalists and the Greens the exit poll suggested it would still be well short of a majority in parliament.
Miliband, who was widely perceived to have performed better in the campaign than expected, is likely to come under pressure to step down as leader.
Indeed if the exit poll is right, three of Britain's political parties could soon be looking for a new leader.