But the president also criticized the withdrawal agreement that Johnson reached with E.U. leaders in Brussels, suggesting it would hinder plans for a bilateral trade deal between the United States and Britain. “To be honest with you, this deal, under certain aspects of the deal, you can’t do it,” Trump said.

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A U.S.-Britain trade deal would allow trade between the two nations to surge, Trump said. “I know Boris wants to be very careful about that,” he added.

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The remarks drew a defensive response from members of Britain’s Conservative government. Housing Secretary Robert Jenrick told BBC Radio 4′s “Today” program that Johnson’s deal would allow Britain to “strike free trade deals around the world.”

Jenrick also said that, despite Trump’s criticism, initial talks with U.S. counterparts about a bilateral trade deal have been “positive.”

Trump was speaking to Nigel Farage, a prominent anti-E.U. campaigner who hosts a radio show on London-based LBC. Farage, the leader of the upstart Brexit Party, has been an ally of Trump’s for years: The British politician spoke at a Trump rally in August 2016, and the president later suggested that Farage should be Britain’s ambassador to Washington.

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The president’s call to Farage came after Britain set the date for its next election on Dec. 12. In his interview Thursday, Trump suggested that Farage’s Brexit Party should team up with Johnson’s Conservative government in the upcoming vote.

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“I’d like to see you and Boris get together, because you would really have some numbers,” Trump said, suggesting that Farage and Johnson would be an “unstoppable” pair. “He respects you a lot. . . . I don’t know if you know that or not.”

In unprompted remarks, Trump also criticized Jeremy Corbyn, leader of the left-leaning opposition Labour Party: “He would be so bad. He would take you in such a bad way. He would take you into such bad places.”

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The Labour leader responded in a tweet later on Thursday. “Donald Trump is trying to interfere in Britain’s election to get his friend Boris Johnson elected,” Corbyn wrote.

Trump has frequently offered his thoughts on Britain’s exit from the E.U., even dubbing himself “Mr. Brexit” in 2016. However, his advice on negotiations has often been vague, and there is little evidence that British voters appreciate it. Trump is unpopular with the British public, according to multiple opinion polls.

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The president has repeatedly emphasized the possibility of a U.S.-British trade deal after Brexit, but many analysts are skeptical of the merits of such a deal. Corbyn has warned of a “Trump-deal Brexit” that would lead to “a one-sided United States trade deal that will put us at the mercy of Donald Trump and the biggest American corporations.”

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In his talk with Farage, Trump dismissed critics who said a trade deal would allow the United States to take advantage of Britain’s beloved National Health Service.

Despite running seven times, Farage has never won a seat in Britain’s Parliament. However, his newly formed Brexit Party performed well in European Parliament elections in May, scoring over 30 percent of the total British vote and taking many votes from the Conservatives, who won 9 percent.

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Farage has not said whether he will run in December’s election, and some British outlets have reported that his party is concerned it might split the pro-Brexit vote and diminish the chances of the Conservatives retaining their hold on the government.

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But the Brexit Party leader suggested in his interview with Trump that he could team up with Johnson only if the British prime minister agreed to walk away from the agreement he reached with E.U. leaders this month. The Brexit Party is expected to unveil its election strategy at a launch party Friday.