LONDON -- President Trump advised British Prime Minister Theresa May to "sue" the European Union in the tense negotiations over Britain's exit from the bloc, May said Sunday.

The American president told reporters Friday at a joint press conference with May that he had given the British leader a suggestion that she found too "brutal." Asked Sunday on the BBC's Andrew Marr Show what that suggestion was, May said with an amused expression: "He told me I should sue the EU. Not go into negotiation, sue them."

She added: "What the president also said at that press conference was, 'Don't walk away. Don't walk away from the negotiations. Then you're stuck.'"

Mr. Trump's first official visit to Britain this week came ahead of a summit with Russian President Vladimir Putin in Helsinki, Finland, on Monday. The U.K. portion of the trip was overshadowed by explosive and undiplomatic remarks he made about May's leadership, especially her handling of the Brexit negotiations.

Get Breaking News Delivered to Your Inbox

In an interview with The Sun newspaper published Thursday -- just as May was hosting Mr. Trump at a lavish black-tie dinner -- Mr. Trump said the British leader's approach likely "killed" chances of a free-trade deal with the United States. He said he had told May how to conduct Brexit negotiations, "but she didn't listen to me."

The comments shocked many in Britain -- even May's opponents. The U.S. president later sought to soften the blow , telling reporters at Friday's joint news conference that May was an "incredible woman" who is "doing a fantastic job" as prime minister.

He denied he criticized May, saying the Sun interview did not focus on his complimentary remarks about the British leader, but the Sun released audio that proved otherwise.

Mr. Trump's comments threatened to weaken May's already fragile hold on power. Her Conservative government is deeply split between supporters of a clean break with the EU and those who want to keep close ties with the bloc, Britain's biggest trading partner.

Brexit Secretary David Davis and Boris Johnson, the former foreign secretary, both quit last week to protest May's trade plan -- and Mr. Trump even said that Johnson would make a good prime minister.

Asked to rate U.S.-U.K. relations, Mr. Trump gave them the "highest level of special." He added it was up to May how to handle Brexit, though he wants to ensure the U.S. "can trade and we don't have any restrictions" on commerce with the United Kingdom.