Boris Johnson has insisted Donald Trump was “patently in error” for claiming the current Brexit deal will hinder the UK’s ability to strike a free trade deal with America.

In an unprecedented intervention – just 48 hours into the election campaign – the US president cast doubt on a future trading pact, claiming his country “can’t make a deal” under the terms of the withdrawal agreement.

Post-Brexit trade with the US is prized by Brexiteers as a major benefit of severing ties with the EU, despite concerns in some quarters that the UK would be forced to deregulate food standards.

“We want to do trade with the UK, and they want to do trade with us,” Mr Trump told the Brexit Party leader Nigel Farage during a radio interview on Thursday.

But he added: “To be honest with you, this deal, under certain aspects of the deal – and you can’t do it, you can’t do it, you can’t trade. We can’t make a trade deal with the UK.”

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Pressed on his remarks on Sky New’s Sophy Ridge on Sunday, the prime minister said: “I don’t wish to cast any aspersions on the president of the United States, but in that respect he is patently in error.”

“Anybody who looks at our deal can see it is a great deal and what it does, is it allow us to take back control of our money, our border and our laws. But also, it allows us to have full unfettered control of our tariff schedules,” he added.

Mr Johnson also said he will apologise for breaking his pledge to deliver Brexit by 31 October, as he described the election as “essential” and the “only way out of the trap parliament had constructed”.

He insisted he was “absolutely” sorry for failing to deliver Brexit by the Halloween deadline and when asked whether he will apologise to Conservative members, who had voted for him on the basis of the pledge, he said: “Of course, of course. It’s a matter of deep regret”.

Pressed on the post-Brexit transition period, the prime minister also claimed he could see “no reason whatsoever” why the government should be forced to extend it beyond December 2020, adding: “If you get the right parliament anything’s possible.