U.S. President Donald Trump attacked British Prime Minister Theresa May and her ambassador to Washington on Monday while Britain voiced regret for the leak of confidential memos in which the diplomat called Trump's administration "dysfunctional" and "inept."

The memos from Kim Darroch, British ambassador to Washington, were divulged to a Sunday newspaper, annoying Trump and embarrassing London.

Darroch had said Trump's presidency could "crash and burn" and "end in disgrace," according to a cache of secret cables and briefing notes sent back to Britain and seen by the Mail.

"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 memo, according to the newspaper.

Darroch is one of Britain's most experienced diplomats whose posting in Washington began in January 2016, prior to Trump winning the presidency.

'What a mess'

Visibly angered by May's continuing support for her ambassador, Trump assailed the prime minister over her handling of fraught Brexit negotiations, and welcomed her impending departure from office.

"What a mess she and her representatives have created," Trump fumed in a series of tweets. "I told her how it should be done, but she decided to go another way."

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

As for London's man in Washington, Trump declared he would have no further contact with him.

"I do not know the Ambassador, but he is not liked or well thought of within the U.S," the president said, doubling down on weekend comments claiming his administration were "not big fans" of Darroch.

"We will no longer deal with him," Trump vowed.

UK offers 'full support'

But even after Trump's remarks, a government spokesperson said that "Sir Kim Darroch continues to have the prime minister's full support." 

The cables' publication in the Mail on Sunday newspaper came just a month after Trump enjoyed a state visit to Britain that included a 41-gun salute welcome at the Buckingham Palace and a banquet dinner with Queen Elizabeth II. 

The incident threatens to complicate London's efforts to strike a new trade agreement with key ally the United States, in hope of mitigating potential damage from Britain's withdrawal from the European Union. 

Trade minister Liam Fox, who is visiting Washington, told BBC radio he would apologize to Trump's daughter Ivanka, whom he was due to meet. 

"I will be apologizing for the fact that either our civil service or elements of our political class have not lived up to the expectations that either we have or the United States has about their behavior, which in this particular case has lapsed in a most extraordinary and unacceptable way," he said. 

"Malicious leaks of this nature ... can actually lead to a damage to that relationship, which can therefore affect our wider security interest." 

'Very serious consequences' for leaker

British officials defended Darroch as carrying out his duties by providing "frank" accounts of developments in Washington.

"Our ambassadors provide honest, unvarnished assessments of politics in their country," a Downing Street spokesperson said.

"As you'd expect, contact has been made with the Trump administration setting out our view that we believe the leak is unacceptable."

British Foreign minister Jeremy Hunt, one of two men who might replace May by the end of the month, said: "I have made it clear that I don't share the ambassador's assessment of either the U.S. administration or relations with the U.S. administration, but I do defend his right to make that frank assessment."

He also promised "serious consequences" for whoever had leaked the memos.

An inquiry was under way to determine who was behind the serious disclosure of confidential material. May's spokesperson said police would be involved if there was evidence of criminality.

The Daily Telegraph newspaper said such memos are seen by up to 100 people working in the Foreign Office and other government departments.

"But it would require a single official or minister to have access to the whole cache, inevitably casting the spotlight on senior ministers," it wrote.

(With input from wires)