President Donald Trump teed off on British Prime Minister Theresa May in an interview published late Thursday, saying her “soft” Brexit plan would “kill” any potential trade deal between the U.S. and the U.K.

“ “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.” ”

In a bombshell interview with the British newspaper The Sun, Trump said May has ignored his advice on how to conduct Brexit negotiations.

“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, she didn’t listen to me. . . . I think what is going on is very unfortunate.”

Trump said a “soft” Brexit that May has backed, in which the U.K. and the European Union would maintain some ties after the U.K. leaves, would “definitely affect trade with the United States, unfortunately in a negative way.”

“No, if they do that I would say that that would probably end a major trade relationship with the United States,” Trump said.

Aside from the EU, the U.S. is Britain’s largest trade partner, with exports worth £100 billion as of 2016, according to the U.K.’s Office for National Statistics, more than twice as much as exports to any other single country.

Read:Brexit turmoil: 4 things investors need to know

May is under fire from within her own party as Brexit negotiations have not gone as planned and the deadline to leave the EU looms. Last week, the minister in charge of Brexit negotiations, David Davis, and foreign minister Boris Johnson both resigned over frustrations with the progress of Brexit negotiations. There is speculation that May’s Conservative Party may oust her as leader and therefore as U.K. prime minister.

Trump further stirred the pot in the interview by saying Johnson would “make a great prime minister.”

Criticizing a foreign leader while visiting their country is considered a major diplomatic faux pas.

In a statement issued after the interview was published, White House press secretary Sarah Sanders said: “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.’”

Also see: Two years after Brexit vote, these charts show the U.K. economy losing momentum

Trump is making his first presidential visit to the U.K., and was attending a dinner with May and other dignitaries when the interview was published. Trump and May met to discuss bilateral issues earlier in the day, and are scheduled to meet again Friday.

In the wide-ranging interview, Trump also ripped London Mayor Sadiq Khan for not being tougher on terrorism, accused the EU of destroying its culture by accepting so many immigrants and said he “feels unwelcome” in London.

Tens of thousands of protesters are expected to march in London on Friday to oppose Trump. That will include a 20-foot-tall “Baby Trump” balloon mocking the president.

“I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London,” Trump, who’s staying outside the city, told The Sun. “I used to love London as a city. I haven’t been there in a long time. But when they make you feel unwelcome, why would I stay there?”

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