Sir Keir Starmer is embroiled in another row over Donald Trump after the Labour mayor of London warned of a “resurgent fascism” on the eve of his inauguration.

Allies of the prime minister slapped down Sir Sadiq Khan, distancing the government from his comments, while the Tories condemned them as “unacceptable”. The latest spat comes at a difficult time for the UK-US relationship.

In what would be an extremely rare move, Mr Trump is considering rejecting Sir Keir’s nominee for UK ambassador to the US, Lord Mandelson.

The Trump team is also still angry Labour staffers travelled to the US to campaign for their rivals, the Democrats, in the presidential race.

Sir Sadiq warned of a “resurgent fascism” and “deeply worrying times, especially if you’re a member of a minority community", in The Observer.

“By exploiting economic concerns and a growing distrust of political and media institutions, these reactionary populists have been able to attract new supporters,” he wrote.

“In Germany, the AfD is on course for a breakthrough in next month’s federal elections. In France, the National Rally is topping presidential polls. And, of course, in the US, Donald Trump is back.

He also hit out at close Trump ally Elon Musk, adding: “A billionaire bully shouldn’t be able to use his social media platform as a propaganda tool to amplify lies and advance the cause of the far right.”

Asked about his comments on the BBC on Sunday, Treasury minister Darren Jones said: “I don’t agree with it. I speak on behalf of the government, and we don’t agree with it.”