Donald Trump has warned Theresa May’s Brexit plan will “kill” any future US trade deal in bombshell comments as the President met the Prime Minister in the UK.

Soon after Trump arrived at Blenheim Palace in Oxfordshire for a gala black tie dinner on Thursday night, The Sun newspaper published an interview with the US President that is likely to embarrass his host.

He told the newspaper that Britain remaining so closely aligned to the European Union under her plans would undermine a UK-US trade pact – which is seen as one of the biggest prizes after quitting the bloc.

In a another swipe, Trump also said he would have done the Brexit negotiations “much differently” and claimed the Prime Minister did not listen to his advice.

Trump’s highly-controversial remarks came at the end of a day in which he had already waded deeply into the Brexit row over May’s Brexit vision.

He had used a Thursday morning press conference in Brussels to attack the Prime Minister’s Brexit plan and highlight Cabinet divisions.

In the Belgium city, the 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.

“If they do that, then their trade deal with the US will probably not be made.”

May’s plan “will definitely affect trade with the United States, unfortunately in a negative way”.

He said: “We have enough difficulty with the European Union.

“We are cracking down right now on the European Union because they have not treated the United States fairly on trading.

“No, if they do that I would say that that would probably end a major trade relationship with the United States.”

Labour MP Emily Thornberry has since come to May’s defence saying it had been “extraordinarily rude of Donald Trump to behave like this”.

“She is his host. What did his mother teach him? This is not the way you behave,” the shadow foreign secretary told ITV’s Good Morning Britain.

Thornberry said Trump’s comments on the PM’s Brexit strategy and his suggestion Johnson would be a “great prime minister” were “rudeness upon rudeness upon rudeness”.

Conservative MP Sarah Wollaston said Trump was “determined to insult” May.

She added: “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.”

The timing of Trump’s comments added to the awkwardness. May had used the Blenheim black tie dinner with political and business leaders to press Trump on the benefits of a free trade deal after Brexit.

Addressing the 100-strong group, the Prime Minister said there was an “unprecedented” opportunity to do a deal that boosted jobs and growth in both countries. 

Trump also heaped praise on Johnson, May’s nemesis who resigned over the ‘soft Brexit’ blueprint on ­Monday, suggesting he would “make a great Prime Minister”.

Asked about Johnson’s comments at a private dinner two weeks ago that Trump “would go in bloody hard” if he was handling Brexit, the President said: “He is right.”

He added: “I would have done it much differently. I actually told Theresa May how to do it but she didn’t agree, she didn’t listen to me.

“She wanted to go a different route. I would actually say that she probably went the opposite way. And that is fine.

“She should negotiate the best way she knows how. But it is too bad what is going on.”

Of Johnson, he said: “I have a lot of respect for Boris. He obviously likes me, and says very good things about me.

“I was very saddened to see he was leaving government and I hope he goes back in at some point. I think he is a great representative for your country.”

Asked if the ex-minister could be in No 10 one day, he replied: “Well I am not pitting one against the other. I am just saying I think he would be a great Prime Minister. I think he’s got what it takes.”

As the interview began to make waves, Trade Secretary Liam Fox tweeted from the Blenheim Palace event about how Trump had talked “so positively about UK and US trade”.

Trump also renewed his criticism of London mayor Sadiq Khan, who he said was doing a “terrible” job running Britain’s capital.

The Labour politician has “done a very bad job on terrorism” by allowing so many migrants to come to the city, the President argued.

Speaking to reporters in Belgium after a fiery Nato Summit, Trump had described the UK as a “hot spot right now with a lot of resignations” and dismissed the Prime Minister’s Chequers plan on the next stage of Brexit.

“I would say Brexit is Brexit,” he told reporters.

“The people voted to break it up so I would imagine that’s what they would do, but maybe they’re taking a different route, I don’t know if that is what they voted for.”

He added that it seemed as if the UK was “getting at least partially involved back with the European Union”.

“I’d like to see them be able to work it out so it could go quickly,” he said.

It comes just days after Trump declined to say whether May should remain in post, said he had “always liked” Boris Johnson, who quit as foreign secretary over the Chequers agreement, and described the UK as being in “turmoil”.

The Trumps landed in Air Force One at Stansted Airport at lunchtime on Thursday to kick-start a four-day working visit to Britain as protests against his trip began.