LONDON — Kim Darroch submitted his resignation on Wednesday as Britain’s ambassador to the United States, following the leak of his candid observations about the Trump administration and the subsequent fierce criticism of him and the British government from President Trump.
“Since the leak of official documents from this embassy there has been a great deal of speculation surrounding my position and the duration of my remaining term as ambassador,” Mr. Darroch said in his resignation letter. “I want to put an end to that speculation. The current situation is making it impossible for me to carry out my role as I would like.”
“Although my posting is not due to end until the end of this year, I believe in the current circumstances the responsible course is to allow the appointment of a new ambassador,” Mr. Darroch wrote.
On Monday, Mr. Trump said the White House would no longer deal with Mr. Darroch after the leak of confidential emails written by the ambassador that had described the Trump administration as “clumsy and inept.”
Mr. Trump also accused Prime Minister Theresa May of ignoring his advice and mismanaging Britain’s tortured efforts to leave the European Union, a departure now delayed at least until the end of October. As for Mr. Darroch, the president described him as “wacky,” a “very stupid guy,” and a “pompous fool.”
The dispute has cast a shadow over ties between London and Washington and has taken center stage in the contest to succeed Mrs. May as prime minister.
During a TV debate Tuesday night, Boris Johnson, the former foreign secretary and favorite to succeed Mrs. May, refused several opportunities to say that he would keep Mr. Darroch in his post until a scheduled departure date in January. He also declined to criticize Mr. Trump and played down the rift.
His comments probably made Mr. Darroch’s position untenable, given that Mr. Johnson is the clear front-runner in the contest and is likely to become prime minister later this month.
By contrast, the rival candidate, Jeremy Hunt, the current foreign secretary, described Mr. Trump’s comments as “unacceptable” and said that he would keep Mr. Darroch in his job.
Mr. Johnson said on Wednesday that he regretted Mr. Darroch’s departure, and that whoever leaked the ambassador’s messages should be “run down, caught and eviscerated.”
Alan Duncan, a minister in the Foreign Office and a supporter of Mr. Hunt, dismissed Mr. Johnson’s statement as “insincere guff.” By failing to support the ambassador, and attempting to ingratiate himself with Mr. Trump, Mr. Johnson had “thrown Kim Darroch under the bus,” Mr. Duncan told the BBC.
With Mr. Johnson intent on Britain leaving the European Union in October, the “special relationship” with Washington is of particular importance given that Mr. Tump has promised a trade agreement with Britain.
“I think the reality was that in light of the last few days his ability to be effective was probably limited,” Vice President Mike Pence’s chief of staff, Marc Short, said Wednesday morning in Washington. “So it was probably the right course.”
The controversy surrounding Mr. Darroch’s assessments has struck some members of the diplomatic corps in Washington as a broader peril: As one of his fellow European ambassadors put it, there was little in his cables that could not be found in their own.
Mr. Darroch’s candid descriptions of the administration as inept and chaotic, of reversed decisions and a mystifying policymaking process, closely mirror daily news reports and the findings of the slew of books about Mr. Trump’s tenure.
But a senior American diplomat noted, before Mr. Darroch’s resignation was announced, that the publication of the WikiLeaks trove of 250,000 diplomatic cables in 2010 had a similar result, with the resignation of the United States Ambassador to Mexico.
In response to Mr. Darroch’s decision, Mrs. May paid tribute to him in Parliament and said that the whole cabinet had rightly given him its support.
“Good government depends on public servants being able to give full and frank advice. I want all our public servants to have the confidence to be able to do that, and I hope the house will reflect on the importance of defending our values and principles, particularly when they are under pressure,” she said.
Jeremy Corbyn, leader of the opposition Labour Party, said of Mr. Darroch, “the comments made about him are beyond unfair and wrong.”
During his tenure, Mr. Darroch was a constant whirlwind of intelligence collection, political analysis, and outreach to the administration and Congress. He organized lunches and dinners for the parade of British officials who move through Washington, from trade ministers to the chiefs of the British intelligence agencies.
Because he had served as Britain’s national security adviser, Mr. Darroch had especially close ties to the American national security apparatus, and frequently played a key role in coordination on matters involving Iran, Russia and Brexit.
His parties, especially an annual New Year’s party at the Embassy, which is decorated with art from Britain’s museum collections, were among Washington’s most sought-after social events. They sometimes included Trump administration officials, including Ivanka Trump and Jared Kushner, the president’s daughter and son-in-law, both of whom are close advisers to the president.
Yet the ambassador’s access to the highest levels of government waned under Mr. Trump. In past administrations, American secretaries of state were often at the British Embassy.
But neither Rex W. Tillerson nor Mike Pompeo dealt as much with Mr. Darroch, and the National Security Council was not as welcoming a place, even to one of Washington’s most crucial ambassadors, as it had been in previous administrations, Democratic and Republican.
The State Department had no immediate comment on Mr. Darroch’s resignation. On Tuesday, the department’s main spokeswoman, Morgan Ortagus, acknowledged that the controversy had implications for Britain’s internal politics.
“There’s clearly an election going on in the United Kingdom,” she said. “We’re going to stay out of that and we will, of course, let the White House speak for the President’s tweets.”
Ms. Ortagus stressed the importance of the relationship between the two nations.
“We have an incredibly special and strategic relationship with the United Kingdom,” she said. “That has gone on for quite a long time and it’s bigger than any individual; it’s bigger than any government. It’s something that has stood the test of time and will continue to do so.”
Simon McDonald, the head civil servant in the British Foreign Office, said he had accepted the resignation “with deep personal regret.”
“Over the last few difficult days you have behaved as you have always behaved over a long and distinguished career, with dignity, professionalism and class. The prime minister, foreign secretary and whole of the public service have stood with you: you were the target of a malicious leak; you were simply doing your job.”