Donald Trump has warned Theresa May her Brexit blueprint will "kill" hopes of any future trade deal with the US.
In an explosive intervention, the US president told The Sun: "If they do a deal like that, we would be dealing with the European Union instead of dealing with the UK, so it will probably kill the deal."
The comments from Mr Trump will send shockwaves through Westminster - and emerged as the prime minister hosted him at a lavish banquet at Blenheim Palace, the birthplace of Sir Winston Churchill.
Mr Trump hit out at the PM's handling of the Brexit negotiations, claiming he "would have done it much differently".
"I told Theresa May how to do it but she didn't agree, she didn't listen to me... Deals that take too long are never good ones," he said.
"If you look, she probably went the opposite way."
Mr Trump said ahead of his visit that he could meet Boris Johnson, who resigned earlier this week over Mrs May's Brexit plan.
And in comments that are unlikely to go down well in Downing Street, the president described the former foreign secretary as a "very talented guy" who he hoped would return to government one day.
Mr Trump said: "I think he is a great representative for your country.
"I think he's got what it takes and I think he has got the right attitude to be a great prime minister."
In a wide-ranging interview he also blamed the mayor of London for high crime rates in the city and failing to tackle terrorism.
He said Sadiq Khan had shown the US disrespect by personally attacking him, while acknowledging that he did not feel welcome in the capital.
Mr Trump and Mr Khan have come to blows before, with the London mayor forced to call Mr Trump's claims about the capital untrue.
In this interview, Mr Trump said: "You have a mayor who has done a terrible job in London. He has done a terrible job.
"I think he has done a bad job on crime, if you look, all of the horrible things going on there, with all of the crime that is being brought in."
The interview emerged as the Trumps dined with the Mays, as well as other members of her recently reshuffled cabinet, including new foreign secretary Jeremy Hunt, at the beginning of Mr Trump's working visit.
It also came just days after Mrs May said she believed she would be implementing plans for a new trade deal with the US during his visit.
On Tuesday, she said: "I'm looking forward to positive discussions with President Trump, there are a lot of issues we'll be talking about.
"Yes, we'll be talking about the trade issues - and we're looking for the time when we leave the European Union - we'll be able in the implementation period to negotiate and sign trade deals with the rest of the world."
Mr Trump's interview has already shocked many MPs, with some calling him "repulsive" and questioning why he should meet the Queen during his visit.
Conservative MP Sarah Wollaston tweeted: "The divisive, dog-whistle rhetoric in his @TheSun interview is repulsive.
"If signing up to the #Trump world view is the price of a deal, it's not worth paying."
Labour's Anna Turley, who represents Redcar, said: "Trump is a racist & disrespects our nation. Why does he get to meet our Queen?"
But Jacob Rees-Mogg told the Wall Street Journal the comments "cut through" the government's line on the Brexit deal.
He said: "The government has been pretending that its document doesn't make trade deals any harder, and Donald Trump has just cut through that, and says it does."
Sarah Sanders, Mr Trump's press secretary, said: "The president likes and respects prime minister May very much.
"As he said in his interview with The Sun, she 'is a very good person' and he 'never said anything bad about her'.
"He thought she was great on NATO today and is a really terrific person.
"He is thankful for the wonderful welcome from the prime minister here in the UK."
Downing Street told the Wall Street Journal it has no response to the interview.