WOODSTOCK, England — That's one way to break up a special relationship.

On his first official visit to Britain as the U.S. president, Donald Trump warned in an interview that Theresa May's new Brexit strategy will "kill" any future trade deal with the U.S., backed her rival Boris Johnson for prime minister, accused the mayor of London of being weak on terrorism and said the whole of Europe is “losing its culture” due to mass immigration.

Trump's caustic remarks to the Sun dropped online partway through a grandiose gala dinner at Blenheim Palace, which May had thrown on Thursday in the president's honor. Things started badly. Trump arrived late, leaving the prime minister and her husband standing alone in front of the palace in silence for six full minutes before he finally rolled up.

As May, her husband Philip, her most senior Downing Street aides and half the Cabinet sat in black tie and ball gowns, just after the PM had delivered a big speech to Trump, his wife Melania and scores of U.K. business leaders on her proud hopes for Anglo-American trade, the Sun's Tom Newton Dunn started tweeting out the top lines from the interview.

Trump said May had ignored his advice on how to negotiate Brexit and "went the opposite way" instead. "I would have done it much differently," Trump told the Sun. "I actually told Theresa May how to do it, but she didn’t agree, didn't listen to me."

Speaking of Johnson, Trump said he was "very surprised and saddened" by the resignation.

He added: "If they do a deal like that, we would be dealing with the European Union instead of dealing with the U.K., so it will probably kill the deal. Because we have enough difficulty with the European Union. We are cracking down right now on the European Union, because they have not treated the United States fairly on trade."

A long-awaited white paper published on Thursday outlining her vision for the U.K.'s future relationship with the EU revealed that the U.K. will formally ask the EU for a post-Brexit “association agreement” including a “free-trade area” for goods, a looser arrangement for financial services, alongside a security partnership and continued membership of many EU agencies. The proposed relationship would require continued close cooperation with the EU, including “regular dialogue between U.K. and EU leaders.” May's Brexit strategy led to the resignations of both her Brexit Secretary David Davis and Foreign Secretary Boris Johnson earlier this week.

Speaking of Johnson, Trump said he was "very surprised and saddened" by the resignation, adding that he hopes Johnson will return to government "at some point," and that he would be a "great prime minister."

Trump took aim at London Mayor Sadiq Khan, who he said has "done a terrible job" on terrorism and crime, and at the capital itself. “I used to love London as a city," Trump said. "I haven’t been there in a long time. But when they make you feel unwelcome, why would I stay there? When I say that, I'm talking about government, because the people of the U.K. agree with me."

Most brutally of all, according to the Washington Post, No. 10 was told about the interview on Thursday, but assured it would be positive.

What's behind the extraordinary intervention? On BBC2’s This Week program on Thursday night, host Andrew Neil asked Nigel Farage whether he had been “winding him up over Brexit.” Farage looked smug and did not confirm or deny. Another theory focuses on a meeting between Trump’s security adviser, John Bolton, with the hard-Brexit ERG group of Tory MPs last month.

In an exercise in damage control, White House spokeswoman Sarah Sanders released a statement on Friday saying “The president likes and respects Prime Minister May very much. As he said in his interview with the Sun she is ‘a very good person’ and he ‘never said anything bad about her.’ He thought she was great on NATO today and a really terrific person. He is thankful for the wonderful welcome from the prime minister here in the U.K.”

The question now is how May will respond. The PM is due to hold extensive talks with Trump on Friday on key strategic issues including defense and security, and the threats posed by Russia, Iran and North Korea, after which she is due to give a joint press conference with Trump.

May could take a leaf out of Hugh Grant's book in the film "Love Actually." But more likely she will suggest she will explain the detail of her Brexit plan to the president, and that she is very confident a trade deal can be done. Trump will agree to listen, they’ll both claim the story was overwritten, and then try to go about their day.

Emma Anderson contributed to this story.

Sign up for POLITICO's London Playbook here.