Britain’s ambassador to Washington has resigned after Donald Trump labelled him “stupid” and “wacky” following the release of confidential memos from the envoy in which he branded the US president’s administration inept.

“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,” Kim Darroch said in his resignation letter on Wednesday.

“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.”

The BBC’s diplomatic correspondent said the ambassador had made the decision after Boris Johnson, the favourite to be the next British prime minister, declined to back him in a televised debate on Tuesday night.

Junior Foreign Office minister Alan Duncan was scathing of Johnson, who could become prime minister later this month if grassroots Conservatives elect him party leader.

“He has basically thrown this fantastic diplomat under the bus to serve his own personal interests,” Mr Duncan said.

“His sort of disregard for Sir Kim Darroch and his refusal to back him was, in my view, pretty contemptible.”

In his confidential memos dating from 2017 to the present, Darroch, 65, had said reports of in-fighting in the White House were “mostly true” and last month described confusion within the administration over Trump’s decision to call off a military strike on Iran.

“We don’t really believe this Administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction-riven; less diplomatically clumsy and inept,” Darroch wrote in one cable.

Outgoing Prime Minister Theresa May said “it is a matter of great regret that (Sir Kim Darroch) has felt it necessary to leave his position” as UK ambassador to the US, saying it is important to “defend our values and principles, particularly when they are under pressure”.

May told parliament that ministers had expressed their backing for Darroch, a career diplomat who was due to leave his post at the end of the year.

“I have told him it is a matter of great regret that he has felt it necessary to leave his position as ambassador to Washington,” she said. “The whole cabinet rightly gave its full support to Sir Kim on Tuesday.”

The souring of relations between Britain and the United States comes at a particularly unwelcome time for London, which is mired in the details of how and when to leave the European Union, a departure currently scheduled for October 31.

Theresa May says "it is a matter of great regret that (Sir Kim Darroch) has felt it necessary to leave his position" as UK ambassador to the US, saying it is important to "defend our values and principles, particularly when they are under pressure" https://t.co/aLysk7nWa2 #PMQs pic.twitter.com/q7dTXn8IFg

— BBC Politics (@BBCPolitics) July 10, 2019

Mrs May said diplomats must be able to give “full and frank advice” and she wants all public servants to have the confidence to do that.

Opposition Leader Jeremy Corbyn also defended Mr Darroch, saying Mr Trump’s comments about him were “beyond unfair and wrong”.

In response to the leak, Mr Trump said he will no longer work with the British ambassador whose leaked memos described the US president as “inept” and “dysfunctional”.

Britain had expressed regret about the leaking of confidential memos from Mr Darroch, but not for the content.

After an initially restrained response, Mr Trump fired back on Twitter saying he “will no longer deal” with the British ambassador.

“I do not know the Ambassador, but he is not liked or well thought of within the U.S. We will no longer deal with him,” Mr Trump tweeted.

His Twitter post also criticised Mrs May about Brexit.

“What a mess she and her representatives have created. I told her how it should be done, but she decided to go another way,” he said.

“The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister. While I thoroughly enjoyed the magnificent State Visit last month, it was the Queen who I was most impressed with!” he wrote.

-with AAP