Boris Johnson holds his dog Dilyn as he leaves a polling station at the Methodist Central Hall in London. Reuters

Britain will leave the European Union on January 31 as planned, Prime Minister Boris Johnson promised on Friday after winning a sweeping election victory.

Condemning more than three years of political wrangling over Brexit, he said: "I will put an end to all that nonsense and we will get Brexit done on time by January 31, no ifs, no buts."

With almost all results declared for the 650-seat parliament, Johnson's Conservative party had secured 362 seats -- its biggest majority since the heyday of Margaret Thatcher in the 1980s.

By contrast the main opposition Labour party endured a terrible night, losing 59 seats to 203, forcing leader Jeremy Corbyn to announce plans for his departure.

The anti-Brexit Liberal Democrats announced they would replaced Jo Swinson as leader after she lost her seat in western Scotland to the Scottish National Party (SNP).

US President Donald Trump on Friday congratulated Boris Johnson on his election victory, saying their two countries were now free to strike a "massive" new trade deal.

"Congratulations to Boris Johnson on his great WIN!", Trump tweeted.

"Britain and the United States will now be free to strike a massive new Trade Deal after BREXIT. This deal has the potential to be far bigger and more lucrative than any deal that could be made with the E.U. Celebrate Boris!"

Some key statements after the exit polls:

02:56:43: France welcomes apparently decisive UK vote: Minister

02:31:47: Minister Gove says if conservatives form a majority, I hope we can get Brexit bill through then heal the country

02:27:40: Labour's Mcdonnell says we'll make decisions on whether he and Corbyn should quit once results are in

02:26:50: Minister Gove says I don't believe a second Independence Referendum would be right for Scotland or the UK

02:26:50: UK PM Johnson says I hope you enjoy a celebration tonight — email to party

02:18:20: Conservatives projected to win 368 seats

02:17:39: Naomi Smith, chief executive of the pro-European campaign group best for Briatin: While this exit poll isn’t promising, the influence of tactical voting at this election has yet to be revealed. Let’s be clear: a majority for Boris Johnson tonight would engulf the UK in a fresh crisis. There is no Brexit deal, only an agreement to talk about a deal in the future.

02:16:55: John Bercow, Former Speaker: That would be a phenomenal victory for the Conservative Party and Boris Johnson will feel completely vindicated with the gamble that he took. That would be an absolutely dramatic victory.

02:16:50: Labour Party spokesperson says we knew it was going to be a challenging election

In the last five national elections, only one exit poll has got the outcome wrong - in 2015 when the poll predicted a hung parliament when in fact the Conservatives won a majority, taking 14 more seats than forecast.

John McDonnell, the second most powerful man in the Labour Party, said the election had been dominated by Brexit which has divided the country since 2016.

European diplomats on Thursday welcomed the clarity an apparently decisive election victory for the Conservative party gave to Britain's stalled withdrawal from the EU, but said it would be challenging to agree a trade deal the end of 2020.

"Clarity is good. But a tall order to move on the future relationship in such a short time," one EU official said.

Exit polls showed the Conservative Party of Boris Johnson would enjoy a very clear majority of 86 seats in the British parliament.

French diplomats said that would allow for a swift passage of the Brexit withdrawal agreement.

"If it's confirmed ... we can assume he'll implement what he said he would do, that is to say Brexit at end-January," the French diplomat said, adding the EU wanted the future relationship with Britain to be as strong as possible.

Johnson told party members to "enjoy a celebration" after an exit poll forecast he was on course to win a majority in Britain's election, giving him the numbers in parliament he needs to deliver Brexit next month.

"I hope you enjoy a celebration tonight," he said in an email to party members. "You powered this campaign. We couldn’t have done it without you."

Retrievers, beagles, poodles and pugs lined up in the rain at Britain's polling stations on Thursday as thousands of voters brought their canine companions to take part in the general election.

On the other hand, pet owners took to Twitter to post photographs of their dogs next to polling station signs at schools and community centres across the country, using the trending hashtag #dogsatpollingstations.

A combo image shows dogs outside polling stations. AP

Among the thousands of pets posted were Rex the Labrador, Gemma the Spaniel and a Rok, dressed up in a Scottish Saltire.

Prime Minister Boris Johnson got into the spirit, nuzzling his dog Dilyn at the polling booth, as did Labour mayor Sadiq Khan, who turned up with his pet Luna.

Not to be outdone, one local council posted a picture of a horse tied up outside a polling station.

The meme provided light relief from a frequently fractious campaign to decide who will win the crucial election with Brexit on the line.

The Electoral Commission says that while dogs are allowed to join their owners, they should be kept on a lead.