Prime Minister Boris Johnson on Friday said the British people had given his Conservative government a "powerful new mandate" to deliver Brexit and unite the country.

Exit polls and early results suggest the Tories are on course for a historic victory in Thursday's election, in which Johnson won his own seat in Uxbridge, west of London, with an increased majority.

The Conservatives were forecast to win a thumping 368 out of 650 seats in parliament — which if confirmed would be the party's biggest majority in three decades — according to the survey published as polls closed.

The pound jumped by about two per cent against the dollar on the projected results of what all sides had painted as the most momentous election in Britain in a generation.

"At this stage it does look as though this one-nation Conservative government has been given a powerful new mandate, to get Brexit done and not just to get Brexit done but to unite this country and to take it forward," Johnson said after winning his seat of Uxbridge.

"I think this will turn out to be a historic election that gives us now, in this new government, the chance to respect the democratic will of the British people, to change this country for the better and to unleash the potential of the entire people of this country."

"That is what we will now do, and if we are lucky enough to be returned as the exit poll seems to suggest then that work will begin [...] today."

Johnson had campaigned relentlessly on the promise to "Get Brexit Done", vowing to end years of political turmoil over Britain's future that has weighed on the economy and sharply divided the nation.

With a large majority of MPs, he will be able to get the divorce deal he struck with Brussels through parliament in time to meet the next Brexit deadline of January 31.

Ratifying the Brexit deal would formalise the end of almost five decades of EU-UK integration, although both sides still need to thrash out a new trade and security agreement.

EU leaders meeting in Brussels for a summit were watching the result closely, where France's European affairs minister, Amelie de Montchalin, was first to welcome the result.

"If the exit poll results are confirmed it should allow for a clear majority, something that has been missing in the United Kingdom for several years," she said.

Labour collapse

Britons had braved winter storms and howling winds as they lined up to cast ballots in Britain's third election in nearly five years.

Johnson's victory, if confirmed by official results, would be the best result for the Conservatives since 1987, when Margaret Thatcher was in Downing Street.

He tweeted his thanks to everyone who voted and supported his party, adding: "We live in the greatest democracy in the world."

By contrast, the exit poll spells disaster for the main opposition Labour party which was projected to win just 191 seats — its worst result since 1935.

Left-wing leader Jeremy Corbyn had promised a second referendum on Brexit, in a bid to appeal to half of British voters who still want to stay in the EU.

But he had focused Labour's campaign on a radical programme of economic change, including nationalising key industries, which failed to woo traditional voters.

"It looks as if Brexit dominated. A lot of this was Brexit fatigue. People just wanted it over and done with," conceded his close aide, John McDonnell.

Corbyn was also personally deeply unpopular and dogged by accusations of sympathising with proscribed terror groups and failing to tackle anti-Semitism within the Labour party.

"We knew he was incapable of leading. He's worse than useless," said former Labour interior minister Alan Johnson.

If confirmed, it will be Labour's fourth successive electoral defeat — and the second under Corbyn — which could see the party out of power until 2024.

Whither Remain?

The anti-Brexit Liberal Democrats were also heading for a disappointing night, forecast to win just 13 seats, one more than they won in the election two years ago.

They performed well in European Parliament elections earlier this year, but new leader Jo Swinson failed to make an impact and experts suggest her promise to reverse Brexit without even a new referendum was unpopular.

Hopes also appeared to be dashed that "Remainers" would vote tactically to secure a second referendum.

By contrast the Scottish National Party (SNP), which wants to stop Brexit and deliver an independent Scotland, was forecast to win 55 of 59 seats available.

Former Scottish Conservative leader Ruth Davidson had pledged to swim naked in Loch Ness on New Year's Eve if the nationalists won 50 seats.

'Softer Brexit?'

Johnson remains a polarising figure in Britain thanks to his leading role in the 2016 EU referendum vote for Brexit, which was marked by anti-immigration, populist rhetoric.

But he has again proved his ability to appeal across political lines, as he did when he was mayor of London — aided by a simple message on Brexit.

Eurosceptic leader Nigel Farage also claimed to have boosted the prime minister's chances by declining to stand candidates for his Brexit Party in Tory-held seats.

The Brexit Party, like its former incarnation UKIP, is not predicted to win any seats.

Johnson has promised to put his Brexit plan to parliament before the Christmas break, although it will not likely be ratified until January.

He has then just 11 months to agree a new partnership with the EU before a post-Brexit transition period ends in December 2020.

But with a comfortable majority in parliament, analysts note he could choose to extend that time and negotiate a closer trade deal than previously envisaged.

"Ironically, this is a freer hand for Johnson to negotiate a softer version of Brexit," said Simon Hix of the London School of Economics.