British Prime Minister Boris Johnson and opposition Labour leader Jeremy Corbyn went head to head over Brexit in the last televised debate before next week's election, with Mr Johnson claiming that Britain would have enough time to strike future trade deals post-Brexit.

Mr Johnson the UK would be able to reach an agreement with the European Union about future trade ties, and reach similar deals with other countries, after its scheduled departure from the EU on 31 January.

"We have ample time to get on and build a new free trade partnership, not just with the EU but with countries around the world," Mr Johnson said.

Mr Corbyn said Mr Johnson's plan to negotiate a trade deal with the United States would take seven years.

Six days before Britain votes in its second national election in less than three years, the debate was seen as the last chance for Mr Corbyn to squeeze Mr Johnson's lead in the polls, which mostly point to victory for the prime minister.

The two leaders set an ill-tempered tone to the debate by battling over how to leave the EU.

Mr Johnson ridiculed Mr Corbyn's support for a new referendum in which Mr Corbyn has said he would remain neutral, while the Labour leader said the prime minister's pledge to "get Brexit done" actually meant seven years of trade talks.

More than three years after Britain voted to leave the EU, the 12 December election will determine when, how and even whether Brexit happens.

Polls show Mr Johnson's governing party is well ahead of Labour.

A Panelbase survey showed the Conservatives extending their lead slightly over the opposition party to nine points, up from eight a week ago.

Mr Johnson, who renegotiated a new divorce deal with the EU in October, has promised to "get Brexit done", a slogan he has repeated constantly during campaigning to try to win over Labour supporters who backed leaving the bloc, and those who are simply fed up of the political haggling over the issue.

Britons voted by 52%-48% in 2016 for Brexit but parliament has been stuck in deadlock over the way forward.

If Mr Johnson wins the majority he says he needs, Britain will leave the EU by 31 January and then seek to enact a trade agreement with the bloc by the end of 2020.

Earlier, Mr Corbyn went on the attack, calling Mr Johnson's promises a fraud and saying his Brexit deal would simply be the start of years of "painful negotiations and broken promises".

He also revealed a leaked government document which he said showed the divorce deal would lead to customs declarations and security checks between Britain and Northern Ireland, a direct contradiction to Mr Johnson's statements that it would not create any barriers between the British province and the mainland.

The confidential Treasury report said the impact of the deal would be higher prices and that 98% of Northern Irish exporters would struggle to bear the additional costs.

"There will be other secret reports like this one in every government department that reveal the disastrous impact of Mr Johnson's damaging deal," Mr Corbyn said.

Mr Johnson said the document was "complete nonsense".

He has mocked the stance adopted by Mr Corbyn, who has said a Labour government would strike another exit deal with the EU to include a customs union, and then hold a second referendum on whether Britons wanted to accept it.

Mr Corbyn has said he will be neutral in that vote.

"Have we sent out search parties to discover Jeremy Corbyn's position on the EU?" Mr Johnson said in a speech to supporters in southeast England.

"If you want to get this thing done, if you want to the country to move forward, you've got to vote Conservative."