Trump slams 'stupid' UK ambassador again

British ambassador to the US Kim Darroch labelled Donald Trump's administration dysfunctional. (AAP)

US President Donald Trump says his administration will no longer deal with the UK's ambassador, but May is standing behind Kim Darroch.

British Prime Minister Theresa May has backed Britain's ambassador to the United States after President Donald Trump cut off contact with him following the leak of diplomatic cables that branded the US administration as "inept".

May stood by Kim Darroch on Tuesday amid the controversy over the release of the documents published in the Mail On Sunday newspaper.

Darroch's forthright views have created awkwardness between two countries who often celebrate having a "special relationship."

As if to underscore the snub, Darroch found himself uninvited to a White House dinner held in honour of the Emir of Qatar on Monday.

While British officials hunted for the culprit behind the leak, Conservative Party Foreign Secretary William Hague said the government was right to back Darroch.

"You can't change an ambassador at the demand of a host country," Hague told the BBC.

"It is their job to give an honest assessment of what is happening in that country."

In a series of tweets, Trump said Darroch was "not liked or well thought of" in the US and also attacked May, accusing her of "making a mess" of Britain's departure from the European Union.

"The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister," Trump tweeted.

"While I thoroughly enjoyed the magnificent state visit last month, it was the Queen who I was most impressed with!"

The tweets ratcheted up the pressure on Britain's government over Darroch, who also has been accused by some Brexit-backing UK politicians of lacking enthusiasm for Britain's departure from the EU.

The journalist who reported the leak, Isabel Oakeshott, is a strong Brexit backer and an ally of Brexit Party leader Nigel Farage, who also is Britain's leading champion of Trump.

Trump has been supportive of Farage and once said he would "do a great job" as an ambassador to Washington.

Farage sidestepped that notion Monday, saying "I'm not a diplomat."

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