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 going to the polls on 12 December following a further delay to its departure from the EU, to 31 January 2020.
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.
Nigel Farage, who campaigned to get the 2016 EU referendum, and who is a friend and supporter of Mr Trump, is also planning to stand, as leader of the Brexit Party.
Speaking on Mr Farage’s LBC radio talk show, Mr Trump said the Brexit Party leader 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.”
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 he was critical of the withdrawal agreement Mr Johnson recently reached with EU leaders – which the prime minister wants to get through Parliament if he wins the general election on 12 December.
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.”
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.”
Kicking off Labour’s general election campaign on Thursday, Mr Corbyn 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.