Donald Trump has criticised Boris Johnson's Brexit deal with the EU, saying it restricts the US's ability to do future trade with the UK.
Speaking to LBC, he said that, without the deal, the two countries could "do many times the numbers" than now.
The US president also took a swipe at Labour leader Jeremy Corbyn, saying he would be "so bad" as prime minister.
Mr Corbyn accused him of "trying to interfere" in the UK general election to boost "his friend Boris Johnson".
The UK is officially going to the polls on 12 December after the early election bill became law when it was given royal assent on Thursday.
It follows a further delay to the UK's departure from the EU, to 31 January 2020.
In August, Mr Trump promised a "very big trade deal" with the UK and predicted that leaving the EU would be like losing "an anchor round the ankle".
But speaking to friend and supporter Nigel Farage on LBC, Mr Trump was critical of the withdrawal agreement Mr Johnson recently reached with EU leaders.
Mr Trump told LBC: "We want to do trade with UK and they want to do trade with us.
"To be honest with you... this deal... under certain aspects of the (Brexit) 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."
Analysis: Trump's election intervention
by Jessica Parker, BBC political correspondent
Diplomatic norms dictate that leaders don't wade into the electoral events of other countries.
But of course this isn't the first time that an American president has decided to cross that particular transatlantic channel.
During the 2016 referendum, Barack Obama said that Brexit would put the UK at the "back of the queue" for trade deals.
In June, Donald Trump offered his views on the Conservative leadership contest.
Now, in this fairly wide-ranging discussion, he's talked about both Boris Johnson's Brexit deal and Jeremy Corbyn's suitability for the role of PM.
But the Labour leader doesn't appear too put out - even retweeting the relevant part of the interview.
The truth is that Mr Corbyn is more than OK with putting some distance between himself and Donald Trump; the US President isn't exactly a poster boy for socialism.
And while his comments on the implications of Boris Johnson's Brexit deal on US-UK trade may cause Downing Street some discomfort, some mystery surrounds exactly where Mr Trump believes difficulties may arise as he didn't elaborate.
The prime minister aims to get his deal through Parliament if he wins the general election.
However, Mr Trump also praised Mr Johnson as "the exact right guy for the times".
In response, a Downing Street spokesman said Mr Johnson's Brexit deal with the EU "ensures that we take back control of our laws, trade, borders and money".
"Under this new deal, the whole of the UK will leave the EU customs union, which means we can strike our own free trade deals around the world from which every part of the UK will benefit."
Mr Trump also said Mr Farage, who leads the Brexit Party and is planning to stand in the general election, and Mr Johnson should "get together" to create "an unstoppable force" in UK politics.
The president, who has previously expressed his backing for Brexit, added: "And Corbyn would be so bad for your country, so bad. He'd take you in such a bad way. He'd take you into such bad places.
"But your country has tremendous potential. It's a great country."
Donald Trump is trying to interfere in Britain’s election to get his friend Boris Johnson elected.— Jeremy Corbyn (@jeremycorbyn) October 31, 2019
It was Trump who said in June the NHS is “on the table”. And he knows if Labour wins US corporations won’t get their hands on it.
Our NHS is not for sale.pic.twitter.com/AUhht3pCgL
Mr Corbyn and Mr Johnson are battling it out for the keys to 10 Downing Street, with the Conservative leader promising to get the UK out of the EU as soon as possible, and the Labour leader promising another referendum.
Kicking off Labour's general election campaign, Mr Corbyn earlier warned a post-Brexit trade deal with Mr Trump's administration would give US companies greater access to the NHS, and allow them to profit from it at UK taxpayers' expense.
The prime minister's planned agreement, he said, would "mean yet more NHS money taken away from patients and handed to shareholders."
However, Mr Trump dismissed the Labour leader's claim, saying: "Not at all. 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."
The UK government has said that, under any future trade deal with the US, it wants protections for the NHS.
Elsewhere, Mr Johnson blamed Mr Corbyn for the delay to Brexit.
He said he was "incredibly frustrated" that the 31 October deadline had to be extended, but a Conservative election win would remove the "logjam".
Both leaders, and those of other parties, are beginning six weeks of campaigning.
It comes as John Bercow's 10-year reign as Speaker of the House of Commons came to an end.
He presided over business in the chamber for the final time before his successor is chosen on Monday.