British Prime Minister Boris Johnson's office has defended his Brexit deal with the EU, after US President Donald Trump warned it would make it impossible for the two nations to strike a future trade agreement.

The president, whose impeachment in the US has moved a stage closer following a key vote in Congress, waded into the British election campaign on Thursday to criticise Johnson's divorce terms with the European bloc.

"This deal [...] you can't do it, you can't trade. We can't make a trade deal with the UK," he said.

But a Downing Street spokesman later said the deal would allow the UK to strike "our own free trade deals around the world from which every part of the UK will benefit".

Trump's comments appear at odds with his previous pledge in September that he was working closely with Johnson to strike a "magnificent trade deal" once Britain left the EU.

The US president also launched a stinging attack on the country's main opposition leader Jeremy Corbyn and urged Johnson to unite with eurosceptic hardliner Nigel Farage, a key figure in the 2016 referendum on European Union membership.

"Corbyn would be so bad for your country," Trump told Farage during a phone interview broadcast on his talkshow on British radio station LBC.

"He'd take you in such a bad way. He'd take you into such bad places."

"I'd like to see you and Boris get together [...] I think it'd be a great thing," Trump added.

Farage, whose new Brexit Party is campaigning for Britain to leave the EU without any deal in place, has urged Johnson to form an electoral alliance but has so far been rebuffed.

He is due to launch his party's election campaign on Friday.

'Trump trying to interfere'

Within minutes of the interview airing, Corbyn shot back on Twitter that "Trump is trying to interfere in Britain's election to get his friend Boris Johnson elected".

Johnson agreed new divorce terms with the bloc's leaders earlier this month, ahead of the country's scheduled departure on Thursday.

But he was unable to push the plan through parliament and instead opted to hold a snap pre-Christmas election, blaming his Labour rivals for the latest Brexit delay and promising to now take the country out by a new January 31 deadline.

"If you vote for us and we get our programme through, which we will [...] we can be out, at the absolute latest, by January next year," Johnson said during a campaign stop Thursday at a hospital.

The Conservative leader is riding high in opinion polls going into the December 12 vote that will be Britain's third in four years.

But he risks a backlash over his unkept "do or die" promise to deliver Brexit on October 31 — and has again set himself up for another potential fall by promising to meet the next deadline.

Pro-EU campaigners and business executives have breathed a sigh of relief that Britain avoided a Halloween Brexit nightmare that could have seen it crash out of the EU after 46 years without a plan.