President Trump pulled no punches in condemning Britain's ambassador to the United States on Sunday, saying "we are not big fans of that man" and asserting that he has "not served the U.K. well," after a leaked diplomatic cable showed the ambassador secretly slamming the White House.

Ambassador Kim Darroch described the Trump administration as "diplomatically clumsy and inept" and said he doubted it would become "substantially more normal," according to the cable.

The memo was one of several documents published by the Mail on Sunday in which Darroch apparently made highly negative statements about the government of Britain's closest ally.

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"We’ve had our little ins and outs with a couple of countries, and I would say that the U.K. -- their ambassador has not served the U.K. well, I can tell you that," Trump told reporters. "We are not big fans of that man, and he has not served the U.K. well. So, I can understand it and I can say things about him, but I won’t bother."

The Trump administration has broken from Britain on key issues such as climate change and preventing Iran from developing nuclear weapons.

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Britain's Foreign Office did not challenge the authenticity of the leaked documents, which covered the period of 2017 to the present, and came to Darroch's defense. It called the leak "mischievous behavior" and said the public has expected diplomats to provide honest assessments of the politics in the countries where they're posted.

The Foreign Office said the leaks would not harm the productive relationship between the British government and the Trump White House. A formal investigation of the leak may be set in motion in the coming days.

The State Department declined to comment on the ambassador's comments.

It is customary for senior British diplomats posted overseas to file straightforward memos to senior ministers and security services analysts back home so political trends and possible threats to British interests could be gauged, but it's unusual for a large number of them to be made public.

"We are not big fans of that man." — President Trump on UK Ambassador Kim Darroch

Justice Secretary David Gauke called the leak "disgraceful" but said Britain "should expect our ambassadors to tell the truth, as they see it."

The memos also characterized Trump's policy on Iran as "incoherent, chaotic." Trump has frustrated European allies by withdrawing the United States from the Obama-era multinational nuclear deal designed to prevent Tehran from obtaining nuclear weapons. The Trump White House has said the deal only enriched a known state sponsor of terror.

Relations between Iran and U.S. allies deteriorated markedly in June, following the downing of a U.S. drone that the U.S. and its allies said was in international airspace and the apparent bombing of two tanker ships near Iranian territorial waters.

Last week, Iranian leaders summoned the British ambassador as Tehran fumed over Britain's Thursday seizure of an Iranian tanker believed to be violating the European Union sanctions by providing crude oil to the Syrian regime.

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Meanwhile, Iran President Hassan Rouhani warned European nations last week that Tehran will "take the next step" in increasing its uranium enrichment closer to weapons-grade levels, in flagrant violation of the nuclear deal.

Darroch, though, seemingly harbored more doubts about the White House's credibility. In one leaked memo, he said he did not believe Trump's public explanation for calling off a planned military strike against Iran last month because of concern about possible civilian casualties. Instead, Darroch asserted it was more likely the strike was canceled at the last minute because Trump felt it would be a liability in the upcoming 2020 presidential election.

Darroch said there were doubts about whether the White House "will ever look competent" and that the only way to communicate with the president was by being simple and blunt.

He said that while Trump had been "dazzled" by British pageantry on a state visit hosted by Queen Elizabeth II in June, the successful visit would not lead to a fundamental shift in Trump's priorities.

"This is still the land of America First," he wrote.

Former Foreign Secretary Malcolm Rifkind said in an interview Sunday that the contents of the leaked cables were not surprising or troublesome but that he was concerned such a large cache of embassy documents had been made available to the newspaper.

"I'm not remotely concerned by what the ambassador said," Rifkind said. "He was doing his job properly and for the most part, I agreed with his comments."

Since the memos and telegrams dated back only to 2017, "not that many people will have had access to all the documents and that might help them trace who was responsible," he said.

Darroch's views may lead to some awkwardness, especially since Trump said shortly after his election in 2016 that Brexiteer Nigel Farage would make an excellent British ambassador to the United States.

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Trump has not hesitated to inject himself into Britain's political fray, repeatedly criticizing Prime Minister Theresa May's Brexit negotiating strategy and praising both Farage and former Foreign Secretary Boris Johnson, a strong contender to become the next prime minister.

Fox News' Lukas Mikelionis and The Associated Press contributed to this report.