In an interview with The Sun, Trump said May’s long-awaited blueprint for Brexit, published this week, would “probably kill” any future trade deal between the U.S. and U.K. He also said May’s arch-rival Boris Johnson—who resigned as foreign secretary on Monday in protest at the blueprint—would “make a great prime minister.”

May’s plan for the U.K.’s exit from the European Union is widely seen as describing a “soft Brexit,” in that it would see the U.K. stay aligned with some European rules in order to maintain the free movement of goods. (It would not, however, permit regulatory alignment nor free movement of services, which are the bedrock of the British economy. The British financial sector is reportedly furious.)

Brexit hardliners such as Johnson and the former Brexit minister David Davis, who resigned just before Johnson did, see May’s plan as a betrayal—they don’t want to be aligned with EU rules at all. And Trump has now openly sided with them against the prime minister.

“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,” the president said. “If they do that, then their trade deal with the U.S. will probably not be made.”

“I actually told Theresa May how to [conduct the Brexit negotiations] but she didn’t agree, she didn’t listen to me,” Trump added. “Deals that take too long are never good ones. When a deal takes so long, they never work out very well.”

He also said May’s plans are “not the deal that was in the [Brexit] referendum.” This is false, as the referendum spelled out no detail whatsoever, consisting only of the question: “Should the United Kingdom remain a member of the European Union or leave the European Union?”

On Johnson, Trump said: “I have a lot of respect for Boris. He obviously likes me, and says very good things about me.”

May responded to Trump’s remarks by saying: “We have come to an agreement at the proposal we’re putting to the European Union which absolutely delivers on the Brexit people voted for.” (Again, the British people did not vote for any particular flavor of Brexit; just for Brexit as a concept.)

Even May’s political opponents—outside her own party—seem to be outraged about Trump’s attack on her.

“She is his host. What did his mother teach him? This is not the way you behave,” said Emily Thornberry, the opposition Labour Party’s shadow foreign secretary, in a TV interview. Thornberry described Trump’s comments as “rudeness upon rudeness upon rudeness,” and urged May to stand up to him.

Trump arrived in the U.K. on Thursday afternoon, after flying in from a tumultuous NATO summit in Brussels. He reportedly threatened to pull the U.S. out of the military alliance unless other NATO members—largely European countries—don’t immediately increase their military spending levels.

After what media described as Trump’s “tantrum,” he told reporters that the other NATO members had “agreed to substantially up their commitment” and everyone was “very happy.” In fact, the other participants merely reiterated what they had already agreed: to increase expenditure to 2% of GDP by 2024.

It also seems that Trump and his entourage saw the NATO visit as a sales opportunity. “The United States by far makes the best military equipment in the world. The best jets, the best missiles, the best guns…everybody wants to buy our equipment. In fact, it’s the question, can they make it? Because they are doing very well. Can they make it for so many people?” the president said.

Meanwhile, Energy Secretary Rick Perry provided what was likely an explanation for Trump’s extraordinary outburst over Germany’s oil and gas deals with Russia. “The United States does not support pipelines such as Nord Stream 2 and a multi-line Turk Stream that will only increase reliance on a single source of supply,” Perry said, according to Politico. “When you’re thinking about a new market provider… we want you to think about America first.”