British Prime Minister Theresa May has "full faith" in the UK's ambassador to Washington but does not agree with all of his assessment of the Trump administration, Downing Street has said.

President Donald Trump hit out at Kim Darroch after the leak of sensitive diplomatic messages describing his "inept" administration.

The president said Mr Darroch had "not served the UK well" and his administration were "not big fans" of the envoy.

Britain’s International Trade Secretary Liam Fox said: "Malicious leaks of this nature are unprofessional. They are unethical. And they are unpatriotic.

"Because they can actually lead to damage to that relationship which can, therefore, affect our security interests.

"I think it is unconscionable that any professional person in either politics or the civil service can behave in this way."

A formal investigation will be carried out to determine how the candid and highly embarrassing messages from the ambassador to the government were leaked.

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Foreign Secretary Jeremy Hunt attempted to limit the damage by stressing the comments were Mr Darroch's "personal opinions" and not the views of the British government.

Mr Trump - who was described as "radiating insecurity" in the assessment - was clearly annoyed by the leak.

He told reporters: "The ambassador has not served the UK well, I can tell you that.

"We are not big fans of that man and he has not served the UK well. So I can understand it, and I can say things about him but I won't bother."

Mr Hunt said the ambassador’s job was to give "frank reports" but the UK's view was that Mr Trump's administration was "highly effective".

He said: "It's really important to say that the ambassador was doing his job as an ambassador, which is to give frank reports and personal opinions about what's happening in the country where he works, and that's his job to send back those reports but they are personal opinions, not the opinions of the British government, not my opinion.

"And we continue to think that under President Trump, the United States administration is both highly effective and the best possible friend of Britain on the international stage."

The documents obtained by the Mail on Sunday detail the ambassador’s assessments of the Trump administration from 2017 to the present.

The diplomatic memos suggest that in order to communicate with the president "you need to make your points simple, even blunt".

In the cache of documents, Mr Darroch gives a scathing assessment of the White House: "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."

He questioned whether the White House "will ever look competent".

Following Mr Trump's state visit to the UK in June, Mr Darroch warned that although the president had been "dazzled" by the pomp and ceremony of the trip, his administration would remain self-interested and "this is still the land of America First".