LONDON — Prime Minister Boris Johnson of Britain spoke with President Trump by phone on Tuesday, and to judge by the dueling summaries of the call provided by the White House and 10 Downing Street, Mr. Johnson and Mr. Trump were involved in two completely different conversations.

The White House said the two leaders pledged to negotiate “a robust bilateral free trade agreement once the United Kingdom leaves the European Union.” Downing Street said nothing about a deal, noting instead that Mr. Johnson urged Mr. Trump to lift American tariffs on Scotch whisky.

Such divergent accounts of a leader-to-leader call are not unheard-of, but the timing of this one, on the eve of Britain’s general election campaign, was telling. It shows just how much of a liability Mr. Trump has become for Mr. Johnson. Once, the prime minister talked up the benefits of having a close friend in the White House; now he is distancing himself from a figure who is radioactive to many Britons.

Mr. Trump was only one of a multitude of headaches for Mr. Johnson on Wednesday, as he kicked off his campaign in an election that will serve as a referendum on his Brexit policy — and was already shaping up as one of the most unpredictable, and consequential, of the post-World War II era in Britain.