In a blow to Mr Johnson on the day he had pledged to take the UK out of the EU, the US President said in a radio interview with Brexit Party leader Nigel Farage: “Under certain aspects of the deal... you can’t do it, you can’t do it, you can’t trade.”

Mr Johnson and Jeremy Corbyn clashed over who was to blame for the failure to leave the EU on schedule.

The Prime Minister insisted his deal was “oven ready” and vowed to deliver Brexit by January at the “absolute latest” despite breaking his ‘do or die’ pledge to take the UK out of the EU by 31 October.

Boris Johnson was endorsed by Boris Johnson. Picture: PA

Mr Corbyn said the Prime Minister alone was to blame for his failure to meet the Halloween deadline.

“There are just too many people who are basically opposed to Brexit, who want to frustrate it,” the Prime Minister told reporters during a visit to a hospital in Cambridge.

“It was the mandate of the people. They voted by a pretty substantial majority to do this and Parliament has simply stood in their way.”

Mr Johnson added: “If you vote for us and we get our programme through - which we will because it’s oven ready, it’s there to go - then we can be out at the absolute latest by January next year.”

But the Labour leader said Mr Johnson had to accept responsibility for his failure to honour the commitment during the Tory leadership campaign.

“He said he would rather be dead in a ditch than delay beyond today. But he has failed. And that failure is his alone,” Mr Corbyn said.

Kicking off Labour’s election campaign with a speech in Battersea, south London, he said the only way to resolve Brexit was through a second referendum.

“We need to take it out of the hands of the politicians and trust the people to have the final say,” he said.

General election 2019: Boris Johnson 'sat' on report about Russian threat to UK ...

“Labour will get Brexit sorted within six months. We’ll let the people decide whether to leave with a sensible deal or remain. That really isn’t complicated.”

However, Mr Corbyn did not say how he would vote in that fresh referendum.

He also declined to say whether he would resign if Labour lose the election, and said it would not be “appropriate” to confirm if the shadow cabinet would be appointed to their corresponding roles in government in the event of a Labour victory.

With the Prime Minister looking to make Brexit one of the key themes of the election, Mr Trump made an unhelpful intervention in an interview broadcast by LBC radio by suggesting the proposed agreement with the EU would prevent a trade deal with the US - one of the government’s key objectives.

“We want to do trade with UK and they want to do trade with us,” the US President said.

“To be honest with you... This deal... under certain aspects of the deal... you can’t do it, you can’t do it, you can’t trade.

“We can’t make a trade deal with the UK because I think we can do many times the numbers that we’re doing right now and certainly much bigger numbers than you are doing under the European Union.”

The President also risked embarrassing Mr Johnson by urging the Conservatives to form an election pact with Mr Farage’s Brexit Party, claiming that together they could form an “unstoppable force”.

More helpfully for the Prime Minister, the US President dismissed Labour’s warnings that a post-Brexit trade deal would open up the NHS to American pharmaceutical companies and private contractors.

At his campaign launch yesterday, Mr Corbyn repeated the claim that the health service would be “up for grabs by US corporations in a one-sided Trump trade sell-out”.

“Not at all,” the US President said. “We wouldn’t even be involved in that, no. It’s not for us to have anything to do with your health care system. No, we’re just talking about trade.”

Mr Trump attacked the Labour leader, saying: “Corbyn would be so bad for your country... he would take it to such bad places”.

Mr Corbyn launched Labour’s general election campaign promising an “ambitious and radical” bid to transform the nation, and cast next month’s poll as a battle between the people and “elites” allied with “born to rule” Boris Johnson.

He hit out at “tax dodgers, bad bosses, big polluters, and billionaire-owned media holding our country back” and called out the likes of media baron Rupert Murdoch and billionaire heir the Duke of Westminster.

The Labour leader also put pressure on other party leaders to publish their tax returns, saying anyone who wanted to serve as Prime Minister or Chancellor of the Exchequer should do so.

Mr Corbyn said: “There are two people who should publish their tax returns, in my opinion, and that is the Chancellor of the Exchequer and the Prime Minister.

“I’m not sure they do, but John McDonnell and I do publish our tax returns in full.”

He added: “In fact, the first time my tax return was published, they realised after they had managed to read my handwriting, that actually I’ve been over-generous and given the Inland Revenue £300 more than I should have done. But that’s OK - I don’t want it back, it’s fine.”

General Election 2019: Donald Trump makes extraordinary intervention criticising Jeremy Corbyn