Prime Minister Boris Johnson made a pitch to Brexit-backing opposition voters Friday alongside a former Labour member of his 2016 referendum team, insisting only a vote for him in next month's election would get Britain out of the EU.

With just under two weeks to go until the December 12 vote, opinion polls suggest the Conservative leader is heading for a comfortable majority in the House of Commons.

Johnson has promised that if this happens, he will take Britain out of the European Union on January 31, and deliver a new trade deal by the end of next year.

But wooing Brexit-backing voters remains crucial and at a press conference in London, Johnson sought to emphasise that the entire "leave" project hung in the balance.

"If there's a Conservative majority government, we can deliver the change that people voted for," he said.

Johnson stood alongside Tory colleague Michael Gove and Gisela Stuart, a former MP with the main opposition Labour party, who together campaigned for Brexit in the 2016 EU referendum.

Stuart urged Labour voters who wanted Brexit to hold their noses and back Johnson, trying to reassure them that this "does not make you a Tory".

"If we don't get this over the hurdle now and get it delivered, Brexit will not happen. Let's be clear about that," she said.

Britons voted by 52 percent to leave the EU in June 2016, but the process has been strung out as MPs battled over exactly how, when and even if the result should be implemented.

Brexit has been delayed three times this year, most recently on October 31, when Johnson was forced to ask the EU to postpone after failing to get his Brexit deal through parliament.