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 the 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”.
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.
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”.
BBC political editor Laura Kuenssberg said: “Failing to back Darroch last night was the first major act of the still hypothetical Johnson premiership that led to his resignation.”