Britain’s US ambassador resigns after Trump spat

But the man tipped to replace her in Downing Street in the coming weeks, former foreign secretary Boris Johnson, declined to explicitly back the diplomat, emphasising instead the need to maintain strong ties with the US.

In a series of confidential cables revealed by a British newspaper at the weekend, Darroch described the Trump administration as “uniquely dysfunctional”.

Trump hit back with a torrent of angry tweets, saying he would not deal with Darroch again, and describing him as a “very stupid guy” and a “pompous fool”.

He also criticised May’s “foolish” policies and said it was a good thing she would soon be replaced.

Darroch has been in Washington since January 2016 and was due to stay on until the end of the year.

But in a statement he said: “The current situation is making it impossible for me to carry out my role as I would like.”

Darroch has received support from across the political spectrum in Britain and after he resigned, Foreign Secretary Jeremy Hunt said: “It should never have come to this.”

Hunt, who is vying with Johnson to replace May as leader of the ruling Conservative party and thus prime minister, on Tuesday condemned Trump’s outbursts as “disrespectful and wrong”.

But Johnson declined several opportunities in a TV debate on Tuesday night to back Britain’s ambassador.

Of Trump’s tweets, he said: “I don’t think it was necessarily the right thing for him to do… but our relationship with the US is of fantastic importance.”

Reports suggest that it was at that point that Darroch decided that he must resign.

Yet Johnson himself has a history with the man who is US president, saying while he was still mayor of London in 2015 that Trump was “out of his mind” for suggesting parts of the UK capital were “so radicalised” that even the police stayed away.

“I think he is betraying a quite stupefying ignorance that makes him frankly unfit to hold the office of president of the United States,” Johnson said of Trump the time.

May expressed “great regret” that Darroch was leaving, saying: “Good government depends on public servants being able to give full and frank advice.”

She emphasised “the importance of defending our values and principles particularly when they are under pressure”.

Scotland’s First Minister Nicola Sturgeon, the leader of the separatist Scottish National Party, said Darroch had been “forced out” for doing his job.

“Boris Johnson’s failure last night to stand up for him — and stand up to the behaviour of Donald Trump — spoke volumes,” she said.

Tom Tugendhat, the Conservative chairman of parliament’s foreign affairs committee, warned the incident could have far-reaching implications.

“If the UK can’t protect diplomatic communications and that costs people their careers, when all they’ve done is to execute the wishes of the government, we will degrade the quality of our envoys, diminish our influence and weaken our country,” he said.

The Foreign Office has launched an investigation into the leak of Darroch’s cables, which comes at a hugely sensitive time in US-UK relations.

Trump was given the red-carpet treatment during a state visit to Britain last month, but there are tensions over his policies towards Iran and China.

As Brexit looms, Britain is also hoping for a free trade deal with the US to mitigate the impact of leaving the European Union.

There has been speculation that the leak was aimed at forcing out Darroch and replacing him with a more Brexit-friendly candidate.

Trump has previously suggested the job go to Nigel Farage, a British eurosceptic politician who now leads the Brexit Party and who, like Johnson, was a prominent voice in the 2016 referendum campaign for Brexit.

On Twitter, Farage said Darroch had made “the right decision”, adding: “Time to put in a non-Remainer who wants a trade deal with America.”

Johnson has previously said he would not work with Farage.