British Prime Minister and Conservative leader Boris Johnson poses holding a fish with Conservative candidate for Great Grimsby Lia Nici (left) during a general election campaign visit to Grimsby Fish Market, northeast England on Monday. Britain will go to the polls on December 12. Photo: AFP

UK Prime Minister Boris Johnson vowed on Wednesday to "fight for every vote" after polls predicted a close finish to Britain's general election aimed at settling the Brexit crisis.

Britons head to the polls for the third time in four years on Thursday, against a backdrop of political deadlock since a 2016 referendum which saw a majority opt to leave the EU.

Parliament repeatedly refused to accept divorce terms that former prime minister Theresa May agreed with Brussels, forcing her out and bringing Johnson into the fray with a vow to deliver.

The former London mayor and foreign minister has been hammering home his "Get Brexit Done" message, to win a majority which would enable him to get the deal approved.

He has vowed to take Britain out of the bloc by January 31.

But a closely watched poll released late Tuesday showed his Conservative party's lead over the main opposition ­Labour party had narrowed.

The YouGov study said the Tories were on course for a 28-seat majority in the 650-seat House of Commons under Britain's first-past-the-post system.

On November 27, it forecast a 68-seat majority.

"The margin of error here could put the final number of Conservative seats from 311 to 367," YouGov said.

The lower end of that range would leave Britain with another hung parliament, where the biggest party does not have a majority, and the very real possibility of Brexit being delayed for years or even cancelled in a second referendum.

It could also end the political career of Johnson - a sharply polarizing figure whose appeal to core Tory voters made him the logical choice to replace the increasingly hapless May.

"This could not be more critical. It could not be tighter," Johnson said while helping load milk bottles onto delivery vehicles on the campaign trail in northern England.

"We're fighting for every vote."

Labour leader Jeremy Corbyn, 70, is a passionate campaigner who confounded pollsters by coming within a whisker of winning the last election in 2017.

He has vowed to implement a radically left-wing program to overhaul public services that have been hit by a decade of austerity caused by the global financial meltdown of 2008-09.

But his vague stance on Brexit and repeated accusations of anti-Semitism in Labour under his watch have weakened his appeal to voters, according to opinion polls.