Prime Minister David Cameron won an emphatic election victory in Britain, overturning predictions that the vote would be the closest in decades to sweep into office for another five years, with his Labour opponents in tatters.

The sterling currency soared on a result that reversed expectations of an indecisive result that could have left Cameron jockeying for power with his Labour rival, Ed Miliband.

Instead, Cameron was due to meet Queen Elizabeth before noon to accept a swift mandate to form a government.

But despite the unexpectedly decisive result, more uncertainty looms over whether Britain will stay in the European Union - and even hold together as a country.

Scottish nationalists swept aside Labour, meaning that Scotland, which voted just a year ago to stay in the United Kingdom, will send just three representatives of major British parties to parliament and be all but shut out of the cabinet. That could revive calls for it to leave Britain.

Cameron's victory also means Britain will face a vote which he has promised on continued membership in the EU. He says he wants to stay in the bloc, but only if he secures changes to its rules in negotiations that have not yet begun.

Cameron returned, smiling, to the prime minister's office in Downing Street early on Friday. With fewer than 100 seats yet to be declared, the only major question remaining was whether his Conservatives would win an overall majority to govern alone, or perhaps invite small parties to join them in government.

The BBC forecast an overall majority for the Conservatives of 328 seats in the 650 seat House of Commons, a result beyond the party's most optimistic forecasts, and their fist majority since 1992.

That would mean Cameron no longer needs the Liberal Democrats, with which he has governed since 2010. The centre-left party was crushed, perhaps reduced to single digits after winning 57 seats five years ago.

Among the stunning results, Ed Balls, in line to be finance minister if Labour had won, lost his seat.

Cameron sounded a conciliatory note, especially towards Scotland, likely to be his first immediate headache.

"Above all 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," Cameron said.

"I want my party - and, I hope, a government I would like to lead - to reclaim a mantle we should never have lost, the mantle of one nation, one United Kingdom."

Sterling gained more than 2 cents against the dollar to rise above $1.55 for the first time since late February, and looked on track to enjoy its biggest one-day gain against the euro since January 2009.

SCOTTISH "TSUNAMI"

With almost all of Scotland's 59 parliamentary seats counted, the Scottish National Party (SNP) had won 56 of them, up from just six five years ago, all but obliterating Labour in one of its historic strongholds.

"We're seeing an electoral tsunami on a gigantic scale," said Alex Salmond, the party's former leader, now elected to represent it in parliament in London.

"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 only one lawmaker.

The United Kingdom includes England, Scotland, Northern Ireland and Wales. England makes up 85 per cent of the population but Scottish politicians elected to parliament in London have historically held important government posts. That will now be impossible with the SNP holding nearly all Scottish seats.

In a body blow to Labour, Douglas Alexander, the party's campaign chief and foreign policy spokesman, lost his seat to a 20-year-old Scottish nationalist student. Scottish Labour leader Jim Murphy was also toppled.

Labour's Miliband is widely expected to resign in the wake of his defeat. A North London Socialist and self-described "geek" who never quite connected with working-class voters, he ran a campaign that was widely seen as better than expected, but was always far behind Cameron in polls that asked voters who they saw as a more credible leader.

"This has clearly been a very disappointing and difficult night for the Labour Party," he told supporters after retaining his own parliamentary seat in Doncaster, northern England.

Liberal Democrat leader Nick Clegg is also expected to resign, after seeing the party humiliated as a response to his decision to join the Conservatives in government five years ago and abandon high profile election pledges.

He won his own seat but called it a "cruel and punishing" night.

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.

The party easily secured the third most votes, but could not translate this to many seats under Britain's system, in which candidates stand for seats in individual districts and a party's overall vote tally is meaningless.

One other loser is the opinion polling industry which is likely to face an inquest over its failure to predict the outcome. Before the election, virtually all opinion polls had shown the Conservatives and Labour neck-and-neck. - Reuters