He told The Sun newspaper that May's plan would "probably kill the deal" as it would mean the US "would be dealing with the European Union" instead of with the UK.

Downing Street has not yet reacted to Trump's remarks, the BBC reported.

May has been making the case for a US free trade deal with Trump, on his first UK visit as president. On Thursday, she said Brexit was an "opportunity" to create growth in the UK and US.

"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 US," she said.

Trump told The Sun that conducted the interview while he was in Brussels for the North Atlantic Treaty Organisation (NATO) summit, that UK's blueprint for its post-Brexit relations with the EU was "a much different deal than the people voted on".

He said he had told May how to do a Brexit deal, but: "She didn't agree, she didn't listen to me... She wanted to go a different route." Trump also said he was "cracking down" on the EU as "they have not treated the US fairly on trading".

Trump praised former British Foreign Secretary Boris Johnson and said he would make a "great Prime Minister", adding "I think he's got what it takes".

The news of his interview with the newspaper broke while he was at a black-tie dinner with May at Blenheim Palace, Oxfordshire on Thursday evening.

After it was published, Whitehouse spokeswoman Sarah Sanders said the President "likes and respects Prime Minister May very much", adding that he had "never said anything bad about her".

The US President along with First Lady Melania Trump were given a red carpet reception after they were earlier greeted by 1,000 protesters outside the US ambassador's residence in Regent's Park.

On Friday, May and Trump will watch a joint counter-terrorism exercise by British and US special forces at a military base. Later they will hold talks with the foreign secretary at Chequers -- the PM's country residence in Buckinghamshire.

The President and his wife will travel to Windsor on Friday afternoon to meet the Queen, before flying to Scotland to spend the weekend at Trump's Turnberry golf resort which will be private.