Sir Kim Darroch has resigned as UK ambassador to the US, amid a row over leaked emails critical of President Trump's administration.
The ambassador was branded "a very stupid guy" by the US president, after emails emerged calling his administration "clumsy and inept".
The Foreign Office praised Sir Kim's "professionalism and class".
The ambassador said he wanted to put an end to speculation, adding the leak had made it "impossible" to do his job.
In a letter to the Foreign Office, Sir Kim said: "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."
"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."