Britain's ambassador to Washington resigned Wednesday after being targeted by US President Donald Trump over the leak of highly critical diplomatic cables that put the allies' relationship on edge.

Outgoing Prime Minister Theresa May had offered her support to Kim Darroch after it was revealed he described the Trump administration as "inept".

But the man tipped to replace her in Downing Street this month, former foreign secretary Boris Johnson, declined to back the diplomat, instead emphasising the need to maintain strong ties with the US.

In a series of confidential cables revealed by a British newspaper at the weekend, Darroch called the Trump administration "uniquely dysfunctional" and said the president was "radiating insecurity".

Trump hit back with a torrent of angry tweets, saying he would not deal with Darroch again, describing him as a "very stupid guy" and a "pompous fool".

He also criticised May's "foolish" policies and said it was a good thing she would soon be replaced.

Darroch has been in Washington since January 2016 and was due to stay until the end of the year.

But in a statement he said: "The current situation is making it impossible for me to carry out my role as I would like."

(Source: AFP)