US President Donald Trump has said Britain may not get a trade deal with the United States if Prime Minister Theresa May goes ahead with her plans as they stand for exiting the European Union.

In an interview with The Sun newspaper, Trump said May had failed to heed his advice by going ahead with a so-called soft Brexit strategy following meetings with her Cabinet last week.

"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," Trump said.

Trump gave the interview during his trip to Brussels for the NATO leaders' summit, shortly before arriving in the UK, where he and May held talks on Friday. The comments were published on Thursday evening while Trump and May attended a prestigious black-tie welcoming dinner at Blenheim Palace in Blenheim, Oxfordshire, England.

At the dinner, before the comments had come out, May said: "As we prepare to leave the European Union, we have an unprecedented opportunity to do more. It's an opportunity to reach a free trade agreement that creates jobs and growth here in the UK and right across the United States."

In the interview, Trump said he had offered advice to May on how to craft the Brexit deal.

"She didn't agree, she didn't listen to me," Trump said. "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."

His comments will be viewed as another setback to May's Brexit plans, which had already been hit by a series of high-profile resignations from her Cabinet during the week, including that of former foreign secretary Boris Johnson, who took issue with her EU exit strategy.

Also in the interview, Trump went on to praise Johnson, saying he would make "a great prime minister" and continued his feud with London Mayor Sadiq Khan, saying he had done a "terrible job" dealing with a series of terror attacks in London last year.

Ahead of the scheduled Trump-May meeting, the White House had noted that Trump said the UK and US relationship is strong.

The UK government had not yet responded to Trump's comments, but when he was questioned by a journalist about the Sun interview on Friday morning, Trump said the US and UK "really have a very good relationship".

Following publication of the interview, White House spokeswoman Sarah Sanders said Trump likes May very much.

Trump and May spent Friday holding talks at the prime minister's country residence of Chequers in Buckinghamshire. They were scheduled to visit Windsor Castle for tea with Queen Elizabeth.

Julian Shea in London contributed to this story.