Sir Kim Darroch has resigned as UK ambassador to the US, as a row over leaked emails critical of President Trump’s administration escalates.
Theresa May said Sir Kim’s departure was “a matter of deep regret” after the ambassador said it was “impossible” for him to continue.
Meanwhile, one minister said Boris Johnson had thrown Sir Kim “under a bus” by not fully supporting him.
President Trump said on Monday that the US would not deal with Sir Kim.
The US president had branded him “a very stupid guy” after emails emerged where the ambassador had called his administration “clumsy and inept”.
In a letter to the Foreign Office, Sir Kim said he wanted to end speculation about his position: “The current situation is making it impossible for me to carry out my role as I would like.
“Although my posting is not due to end until the end of this year, I believe in the current circumstances the responsible course is to allow the appointment of a new ambassador.”
The leak was described as “malicious” by head of the diplomatic service Sir Simon McDonald, who told Sir Kim: “You are the best of us.”
He told the Commons’ foreign affairs committee it was the first time in his career that a head of state had refused to work with a British ambassador.
Mrs May said Sir Kim had had the full backing of the cabinet and he was owed an “enormous debt of gratitude” for his “lifetime of service” to the UK.
Public servants should be able to give “full and frank advice”, she said, adding that it was important to defend “our values and principles, particularly when they are under pressure”.
His resignation has prompted widespread support for Sir Kim while some have questioned Tory frontrunner Boris Johnson’s stance.
BBC diplomatic correspondent James Landale said he understood Sir Kim decided to resign after watching Mr Johnson refuse to support him during the Tory leadership debate on Tuesday night.
Mr Johnson was asked repeatedly by fellow leadership candidate Jeremy Hunt whether he would keep Sir Kim in post if he became prime minister, but refused to answer.
Following Sir Kim’s resignation, Mr Johnson said he was “a superb diplomat” and whoever was responsible for the leak “has done a grave disservice to our civil servants”.
Asked why he was not more supportive of Sir Kim, he said it was “wrong to drag civil servants into the political arena”.
This is a truly incredible situation. Sir Kim was one of the UK’s most highly respected diplomats and for him to be forced out like this is a major event.
Traditionally, we’ve had a clear separation between diplomacy and politics, but not now.
A senior government source said to me earlier this week that this country would never let another country decide who represents us abroad – well, that’s what has happened.
Make no mistake, it’s a big convention that’s been broken. And these things matter.
In terms of the leadership race, there’s no doubt that seeing Mr Johnson fail to back him was what changed the dial for Sir Kim and led him to decide he had to quit.
Mr Johnson hasn’t even moved into No 10 yet – the widely held assumption is that he will – but this is the first major act of a still hypothetical premiership.
And if and when he does become PM, this issue could be something that dogs him for some time, and gives those in the party who are hostile to him ammunition.
Europe Minister Sir Alan Duncan – who backs Mr Hunt in the leadership contest – said it was “contemptible negligence” not to support Sir Kim.
“He’s basically thrown this fantastic diplomat under a bus to serve his own personal interests,” he said.
Former foreign office minister Alistair Burt – also a supporter of Mr Hunt – said, referring to Tuesday’s debate: “Anyone, I think, would have seen last night’s events and seen a potential prime minister letting someone go very publicly.”
Tory MP and chairman of the Commons’ foreign affairs committee Tom Tugendhat said in a tweet: “Leaders stand up for their men. They encourage them to try and defend them when they fail.”
However, Sir Michael Fallon – a supporter of Mr Johnson – said the Tory leadership hopeful had made it “very clear that the relationship with the United States is what comes first”.
Fellow Tory leadership candidate and Foreign Secretary Mr Hunt told the BBC Sir Kim was “doing his job” and his resignation was “a black day for British diplomacy”.
President Trump could well wake up this morning thinking he has the power to veto who the UK has as its ambassador.
It wasn’t his more colourful remarks on Twitter that really ended Sir Kim’s time, but Mr Trump’s public announcement that he would no longer work with him.
The effects of that were felt immediately. There was a banquet that Sir Kim was immediately dis-invited from. Next, he couldn’t attend an event with minister Liam Fox.
It was clear he was being frozen out and for an ambassador access is everything. Without it, it’s impossible to do the job.
More broadly, it’s like this… There’s never been parity in the special relationship between the UK and US – it’s never been a relationship of equals but right now it seems particularly lopsided.
The US knows that Britain is fairly isolated right now internationally and needs the US more than ever. Donald Trump has wielded that power mercilessly in this row.
Labour leader Jeremy Corbyn said Mr Johnson wanted a “sweetheart trade deal” with the US and his lack of support for Sir Kim “shows he won’t stand up to Donald Trump”.
In a letter to Sir Kim, Cabinet Secretary and civil service head Sir Mark Sedwill said that while he understood his reasons for resigning it was “a matter of enormous regret that you were put in this position after a shocking betrayal of trust”.
Cabinet Office Minister David Lidington said he was “enraged” by the situation and morale in the senior ranks of the civil service had taken “a very heavy blow”.
In the emails leaked to the Mail on Sunday, Sir Kim said: “We don’t really believe this administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction-riven; less diplomatically clumsy and inept.”
The emails, dating from 2017, said rumours of “infighting and chaos” in the White House were mostly true.
The government has opened an internal inquiry into the publication of the memos and police have been urged to open a criminal investigation.
Downing Street confirmed there had been some “initial discussions” with police regarding the leak and if there was concern about criminal activity they would become involved “more formally”.
Metropolitan Police Commissioner Cressida Dick said it was “deeply worrying” diplomatic cables had ended up in the public domain.