Sir Kim Darroch is to resign as British ambassador to the United States following the leak of memos highly critical of Donald Trump's White House.
In a letter to the head of the Foreign Office, Sir Simon McDonald, Sir Kim said he believed it was "impossible" for him to carry on in his current role.
"Since the leak of official documents from this embassy there has been a great deal of speculation surrounding my position and the duration of my remaining term as ambassador," he said.
"I want to put an end to that speculation. 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."
His decision came after President Trump said the White House would no longer have any dealings with him following the leak of the dispatches describing his administration as "inept" and "dysfunctional".
In a televised debate on Tuesday, Tory leadership frontrunner Boris Johnson repeatedly refused to say whether he would keep Sir Kim in post if he became prime minister.