Britain's ambassador to the US, Sir Kim Darroch, resigned on Wednesday amid an escalating diplomatic row over leaked emails critical of US President Donald Trump's administration.
Darroch, who was the target of personal Twitter attacks as a "very stupid guy" from Trump, said he wanted to put an end to speculation and step down as the leak had made it "impossible" for him to do his job.
"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," he said in his resignation letter addressed to Sir Simon McDonald, the senior-most official at the UK Foreign Office and head of the diplomatic service.
"The current situation is making it impossible for me to carry out my role as I would like," he said.
It comes after a series of diplomatic messages sent by him were leaked at the weekend, in which he described Trump and his administration as "incompetent", "inept" and "insecure".
In his reply, McDonald said Darroch had been the target of a malicious leak and expressed gratitude for his work. "You are the best of us," he said.
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The outgoing diplomat, who was set to retire at the end of the year, also received the backing of Prime Minister Theresa May and foreign secretary Jeremy Hunt, currently in the midst of a leadership contest with Boris Johnson to replace May.
May told MPs in the Commons: "Sir Kim has given a lifetime of service to the UK and we owe him an enormous debt of gratitude. Good government depends on public servants being able to give full and frank advice.
"I want all our public servants to have the confidence to be able to do that, and I hope the house will reflect on the importance of defending our values and principles, particularly when they are under pressure."
Darroch received widespread backing across party lines in the UK, with Labour leader Jeremy Corbyn telling the Commons: "The comments made about him are beyond unfair and wrong. I think he has given honourable and good service and he should be thanked for it."