LONDON — The U.K.'s top diplomat in the U.S. views President Donald Trump as "inept," "insecure" and "incompetent," according to leaked diplomatic cables.

Kim Darroch, Britain's ambassador to Washington, D.C., made the highly critical comments about the president and his administration in a series of memos to London.

NBC News has confirmed the authenticity of the documents. In a statement, a spokesperson for the British Foreign and Commonwealth Office said "a formal leak investigation will now be initiated."

The leaks provide a rare insight into how a key U.S. ally views the Trump administration behind closed doors.

The memos were critical of Trump's economic policies, claiming they could wreck the world trade system, described conflicts within the White House as "knife fights" and warned "the worst cannot be ruled out" in regard to allegations of Trump's collusion with Russia.

"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 of a series of leaked documents covering the period from 2017 to the present.

“The ambassador has not served the U.K. well," Trump told reporters Sunday as he left his golf resort in Bedminster, New Jersey. "I can tell you that. We’re not, we’re not big fans of that man.”

NBC News has reached out to the U.K. ambassador's office for comment.

The U.K. foreign office defended Darroch in a statement, saying that ambassadors are allowed to be candid about their views.

"The British public would expect our Ambassadors to provide Ministers with an honest, unvarnished assessment of the politics in their country," the statement said, adding an ambassador's perspective is not representative of the views of the government.

The statement did not deny the veracity of the leak, instead condemning the apparent breach of protocol.

"It's important that our Ambassadors can offer their advice and for it remain confidential," the statement said.

"Our team in Washington have strong relations with the White House and no doubt that these will withstand such mischievous behaviour.”

Kim Darroch, United Kingdom's ambassador to the United States, speaks during a television interview in New York on May 18, 2018. Victor J. Blue / Bloomberg via Getty Images file

Darroch, a veteran diplomat, is into his third year of what is usually a four-year posting that holds significant weight in U.S.-U.K. relations. He is viewed in Washington foreign diplomacy circles as an effective diplomat, but whether the role fits the agenda of Britain's future prime minister remains to be seen.

The ruling Conservative Party is in the midst of a leadership race.

The U.K.'s foreign secretary, Jeremy Hunt, who is vying to become the next prime minister, said in comments made on camera that Darroch was not speaking for all of the British government.

"We continue to think that under President Trump the U.S. administration is not just highly effective but the best possible friend of the United Kingdom on the international stage," Hunt said.

The leaks, meanwhile, could spark a reaction from Trump, who hasn't always taken kindly to criticism.

Trump has previously lambasted London Mayor Sadiq Khan on Twitter, calling him a “stone cold loser” who was doing a “terrible job” in response to Khan’s condemnation of Trump's state visit to the U.K.

Darroch reportedly referenced the incident while warning officials of more such outbursts.

"For a man who has risen to the highest office on the planet, President Trump radiates insecurity," he said, according to the Mail on Sunday newspaper.

Interactions between Washington and Westminster have been tumultuous throughout Trump's tenure, despite the countries’ much-lauded special relationship.

Trump previously criticized outgoing Prime Minister Theresa May's handling of Brexit while praising her likely successor, Boris Johnson.

Johnson is the favorite over Hunt for the job in a leadership election of Britain's ruling Conservative Party, after May announced her resignation earlier this year.

The winner will be announced later this month.

"I think Boris would do a very good job. I think he would be excellent," Trump told the British tabloid The Sun on the eve of his state visit.

Trump has previously suggested the ambassador’s role be filled by Brexit leader Nigel Farage. He also met with Farage in London during last month's trip.

Farage responded to the leaked cables on Sunday, calling for Darroch to resign or be fired.

"Kim Darroch is totally unsuitable for the job and the sooner he is gone the better," he said on Twitter.

Linda Givetash reported from London and Andrea Mitchell from Washington, D.C.