The opposition Labour leader, Jeremy Corbyn, is using Mr. Johnson’s relationship with Mr. Trump to capitalize on the president’s unpopularity in Britain and claim that the prime minister would sell out the country’s National Health Service for a trade deal.

Mr. Corbyn has good reason to do so: Only 19 percent of Britons surveyed said they had a positive opinion of Mr. Trump, according to YouGov.

Context: Although Mr. Johnson’s Conservative Party leads the Labour Party in the polls, the prime minister was hit by the resignation of one of his cabinet ministers in a legal scandal and accusations that his party doctored a TV interview with a Labour leader.

Quotable: “Donald Trump is so exceptional. We hear and see so much about him that he has almost become a presence in domestic British politics,” said one professor of politics in London.