In an extraordinary intervention in British domestic politics, the US president told reporters at the Nato summit that her proposals would leave the UK “partially involved with the EU”.
The comments were immediately greeted with delight by Nigel Farage, who tweeted: “The president is absolutely right.”
The explosive remarks – even before Mr Trump arrives in the UK – will fuel Downing Street fears that his visit will have an impact on her attempts to calm Tory anger over Brexit.
At a press conference before leaving Brussels, Mr Trump said: “Brexit is Brexit. The people voted to break it up, so I would imagine that is what they’ll do, but maybe they are taking a different route. I don’t know if that’s what they voted for.”
And he added: “Brexit is turning a little differently – they seem to be getting involved at least partially with the EU.”
“I’d like to see them be able to work it out so it could go quickly,” Mr Trump also said.
The president also played down the huge protests expected to accompany his visit, claiming: “I think that those people like me a lot and they agree with me on immigration.
“I think that’s why you had Brexit in the first place – because of immigration.”
Mr Trump also said, of the state of British politics: “I’m going to a pretty hotspot right now, with a lot of resignations.”
On Brexit, he added: “I’ve been reading a lot about Brexit and it seems to be going a little differently recently.
“It’s not for me to say, I own a lot of property there and I’m going to Turnberry in Scotland, which is a magical place.”
Mr Trump’s fresh intervention comes just days after he declined to say whether Ms May should remain in post and described the UK as being in “turmoil”.
He arrives in the UK after causing a firestorm at the Nato summit in Brussels when he threatened to pull the US out of the alliance unless other nations stumped up extra cash.
The president backed down over the threats when other leaders agreed to increase defence spending “like they never have before” and described himself as a “very stable genius” over the deal.
The prime minister will host Mr Trump and his wife Melania at a lavish dinner at Blenheim Palace on Thursday evening.
After spending the night at the US ambassador’s official residence in Regent’s Park, London, he will join Ms May at a military base on Friday to observe a joint counter-terrorism exercise involving UK and US special forces.
The two leaders will hold talks at the prime minister’s country residence of Chequers where Russia, Brexit and the Middle East will top the agenda.
Mr Trump will then travel to Windsor Castle to meet the Queen before heading to Scotland for a private part of the four-day visit, ahead of travelling to Helsinki for a summit with Russian president Vladimir Putin.