US President Donald Trump says a free trade deal with Britain might be impossible if the country goes ahead with Prime Minister Theresa May’s proposals for post-Brexit ties with the European Union, in a damaging intervention set to further criticism of her plans.

In an interview with The Sun newspaper, published on Friday morning, Mr Trump said Ms May’s plans for a business-friendly Brexit would leave it too close to the EU to allow a new trans-Atlantic trade deal to be struck.

The publication came after Mr Trump and First Lady Melania Trump arrived in England, hours after meeting Ms May at a lavish dinner to mark his first visit to Britain as US president, amid multiple protests across the country.

Ms May made a direct pitch for a trade deal with Washington, praising the friendship between the two allies, glossing over Mr Trump’s previous remarks that Britain was a “hot spot” in turmoil over Brexit.

“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,” Mr Trump told The Sun, referring to Ms May’s Brexit proposals.

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

Melania Trump, Mr Trump and Ms May at Blenheim Palace. Photo: Getty

Mr Trump said he had told Ms May how to do a Brexit deal, but the Prime Minister did not heed his advice.

“She didn’t agree, she didn’t listen to me,” he told The Sun.

“I told her how to do it. That will be up to her to say. But I told her how to do it. She wanted to go a different route.”

Mr Trump also said he was “cracking down” on the EU as “they have not treated the United States fairly on trading”.

He added that former foreign secretary Boris Johnson would make a “great prime minister”, saying “I think he’s got what it takes”.

The Sun said its interview with Mr Trump was conducted in Brussels before he left for the British leg of his trip.

Bombshell for May already battling Brexit pains

Mr Trump’s intervention comes at the end of a tumultuous few days for Ms May. Two senior ministers including Mr Johnson resigned in protest this week at her plans for trade with the EU after Britain leaves the bloc next March.

Tomorrow's front page: Donald Trump @realDonaldTrump accuses the PM of wrecking Brexit – and warns she may have killed off any chance of a vital US trade deal – full story HERE at 11pm https://t.co/JRrMjQDTBq pic.twitter.com/udCjYWQeeQ

— The Sun (@TheSun) July 12, 2018

Ms May’s “business-friendly” Brexit plan — which would keep Britain in a free trade zone for goods with the EU but mean it has to share some EU rules — was agreed by her cabinet only last Friday after two years of wrangling since Britons voted to leave the bloc in a 2016 referendum.

A trade deal with the United States is one of the main aims of Brexit supporters within Ms May’s Conservative Party, who are concerned that she is making too many concessions to the EU.

Some Brexit supporters have cast May’s Brexit plan as a betrayal, including MPs in her own deeply-divided party who have warned of a leadership challenge.

Before the Sun interview was published, Ms May invoked Winston Churchill as she addressed Mr Trump and business leaders at a black-tie dinner at Blenheim Palace, the grand 18th-century country house which was the British World War II leader’s birthplace.

“Mr President, Sir Winston Churchill once said that, ‘To have the United States at our side was, to me, the greatest joy’,” Ms May told Mr Trump, according to a text of her speech provided by her office.

“The spirit of friendship and cooperation between our countries, our leaders and our people, that most special of relationships, has a long and proud history.

“Now, for the benefit of all our people, let us work together to build a more prosperous future.”

Mr Trump had already cast doubt on Ms May’s Brexit plans earlier in the day after a NATO summit in Brussels, where he provoked a crisis session to force allies to raise their defence spending.

Whilst Mr Trump’s trip is not the full state visit he was originally promised, he was heralded by military bands on his arrival in the country and at Blenheim, and later today he will have tea with Queen Elizabeth.

He will spend the weekend at one of his golf resorts in Scotland.

Protests against Trump upon arrival in England

Demonstrators protest near the US ambassador’s residence in London. Photo: Getty

Demonstrators lined the streets outside Blenheim Palace in Oxfordshire where the president and first lady attended the black-tie dinner.

But it is unlikely the Trumps even saw them. Arriving via helicopter, the couple was then driven by presidential limousine onto the property, where they were welcomed by Ms May and her husband Philip.

Among the crowd of protesters outside, American man Martin Williams, who works at Oxford University, called Mr Trump a dictator.

“He’s a racist that’s taken over my country. He’s a wannabe dictator. He is imprisoning children and tearing them away from their families. I mean, the list goes on,” he said.

“He is destroying all the norms of what used to be a great democracy in America and so I’m really concerned about my country and I’m here now and so this is where I have the chance to protest him.”

The biggest protests are set for Friday, when demonstrators plan to march through central London and float a huge balloon depicting Mr Trump as a baby in a nappy over the Houses of Parliament.

The National Police Chiefs Council said rest days for officers had been cancelled and many would be working 12-hour shifts during Mr Trump’s visit.

-ABC