Donald Trump issued scathing criticism of Prime Minister Theresa May's Brexit strategy during his visit to Britain, as campaigners planned a mass protest against the US president on Friday.
In a series of extraordinary broadsides, Trump told Friday's edition of The Sun that May's plan for post-Brexit ties with the EU would "probably kill" the prospects for a trade deal with the US.
Trump also said former foreign minister Boris Johnson, who resigned this week in protest over the plan, would make "a great prime minister".
He said London mayor Sadiq Khan, who has allowed protesters to raise a giant diaper-clad balloon mocking the US president next to parliament on Friday, was doing "a terrible job" on crime.
The comments will plunge the transatlantic "special relationship" to new lows, making for extremely awkward talks with May when the two meet at her country retreat of Chequers later on Friday.
As he began a four-day visit to Britain on Thursday, Trump, a major supporter of Brexit, told The Sun that he had advised May to go about leaving the EU in a different way but was ignored.
"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.
May has insisted that her Brexit plan will allow Britain to strike its own lucrative trade deals with countries like the US after her country has left the bloc as planned in March next year.
Brexit "creates an opportunity to reach a free trade agreement that creates jobs and growth here in the UK and right across the United States", she said.
But Trump poured cold water on May's offer.
"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," he said.
Trump flew into Britain on Thursday, having already subjected Nato allies to a roasting over their levels of defence spending.
The president spent the night at the US ambassador's residence located in a London park behind a "ring of steel" fencing, stalked by noisy demonstrators.
White House spokeswoman Sarah Sanders later told reporters Trump was "thankful for the wonderful welcome" he received in Britain. (AFP)