Labour leader Jeremy Corbyn also seemed ready to head to the polls, releasing a campaign video after the vote calling for “real change”.

He is pitching a radical socialist government and another European Union referendum.

The UK has so far been unable to complete its tortured divorce from the EU, and Johnson has failed to get Parliament to ratify his withdrawal agreement.

The election may now turn into a proxy referendum on Brexit, the last chance for voters to choose between politicians campaigning to stay in the EU or leave it immediately.

The polls have Johnson far ahead: YouGov had 36 per cent of voters backing his Conservatives, with Labour in second place on 23 per cent.

But his move is still a risky one, and could backfire. Defeat would consign Johnson to the history books as one of Britain’s worst prime ministers.

A December vote is a rare event in Britain – the last was held in 1923. Critics have cited difficulties in reaching voters in the run-up to the Christmas holidays, poorer weather and darkness as obstacles.

The last time the country went to the polls, in 2017, Labour and the Conservatives won more than 80 per cent of the vote between them as the smaller parties faded away.

Jeremy Corbyn: Britain’s saviour or great white nope?

This time, with Brexit still unresolved, both main parties face challenges from marginal movements that have grown in popularity.

On the pro-EU side, the Liberal Democrats have surged in opinion polls and lower profile elections recently, on their platform of clear opposition to Brexit.

That is a threat to Corbyn, in particular. Many of his supporters want him to adopt a clearer anti-Brexit stance.

For Johnson, the main peril comes from the veteran Eurosceptic Nigel Farage.

He launched his Brexit Party in January to campaign for a clean, quick split from the EU. It then won popular vote in the European Parliament elections in May.

With Johnson having failed to deliver his core mission to complete Brexit by October 31, the door is open to Farage to take votes away from the Conservatives.

Steve Bannon says Brexiteer ‘friend’ Nigel Farage in the running to be UK prime minister

On Monday, the EU agreed to postpone Brexit day until January 31 to give Johnson more time to persuade members of Parliament to ratify the deal he struck on October 17. That gave Corbyn the justification he needed to back a poll.

“For the next three months, our condition of taking no-deal off the table has now been met,” Corbyn told his top team, according to a party statement.

“We will now launch the most ambitious and radical campaign for real change our country has ever seen.”

At a record-breaking 71-minute question-and-answer session with MPs on Wednesday, Johnson said the “only way to deliver a great Brexit is to vote for this party and for this government”.

He accused the socialist Labour leader of pushing an “economically disastrous” re-nationalisation programme and “continually to flip-flop one way or the other” on Brexit.

Corbyn shot back by claiming that the Conservative prime minister’s post-Brexit trade proposals were selling out British interests to US President Donald Trump.

“Despite his denials, [the National Health Service] is up for grabs by US corporations in a Trump-style trade deal,” Corbyn said.

Corbyn’s attack line underscores Labour’s desire to shift the campaign’s focus to traditional social issues and away from its past difficulties with the defining issue of UK politics.

Did Queen Elizabeth’s ‘EU hat’ carry coded Brexit message?

The Conservatives are pinning a lot of their hopes on Johnson’s undoubted fame. Unlike his predecessor Theresa May, he enjoys campaigning. But that fame brings a problem. Most people have made up their minds what they think of Johnson, and a lot of them don’t like him.

According to YouGov, 47 per cent of people have a negative opinion of him, against 33 per cent who have a positive one. Labour is pushing hard on Johnson’s tendency to go back on promises.

John Curtice, a politics professor at Strathclyde University, said that if Johnson maintained his lead until Election Day – and that’s a big if – “that should be enough to deliver an overall majority, not necessarily an overwhelming one, but probably enough for him to be able to do what he wants to do on Brexit”.

But Curtice noted that if Johnson’s Conservatives don’t win an overall majority, they have fewer friends than Labour with which to broker deals.

Paula Surridge, a political sociologist at the University of Bristol, said the December 12 election was “likely to pose key challenges to both the Labour and Conservative parties”.

Can UK’s Liberal Democrats stop Brexit?

“Who manages to hold on to the larger part of the 2017 votes is likely to form the next government. Will this be the Brexit election 2017 never quite managed to be?

“The Liberal Democrats and the Brexit party will be pushing hard to make it so, while Labour are likely to want to talk about almost anything else.”

Bloomberg, The Washington Post, The Guardian, Reuters