LONDON - 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.
Sterling GBP=D4 rallied to its highest in just over a week in early Asian trade on Friday, jumping to a high of $1.5448 and outperforming its peers.
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.