President Donald Trump stepped up his attacks on Britain's ambassador to the United States on Tuesday as Prime Minister Theresa May reaffirmed her support for the diplomat whose leaked emails called Trump's administration "inept."

In a series of tweets, Trump branded Kim Darroch a "wacky Ambassador that the U.K. foisted upon the United States," adding that he was "not someone we are thrilled with, a very stupid guy." May's office described the email leaks as "unfortunate" but insisted that Darroch would continue to carry out his diplomatic duties for the time being.

The confidential memos leaked Sunday to Britain's Daily Mail newspaper contained stinging criticisms of Trump's White House, portraying it as "clumsy" and completely absorbed by "infighting and chaos." The emails date back to 2017. Trump said Monday he would no longer deal with Darroch. He called him "insecure" and "incompetent."

The latest tweets from the president went a step further as he labeled Darroch a "pompous fool." Trump claimed to not know the ambassador but he has met him a few times, including recently during his state visit to Britain. Darroch was disinvited to a dinner with Trump and Qatar's emir on Monday, according to U.S. and British officials.

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The escalating furor comes at an awkward moment for Britain, which is in the process of selecting a new prime minister, after May steps down in a few weeks. The British government has launched a formal investigation into the source of the leaks but speculation in the British press has suggested, without evidence, that the leaker may have been motivated by wanting to see a more pro-Trump, pro-Brexit envoy in Washington representing Britain's economic, cultural and political interests.

"It was clearly somebody who set out deliberately to sabotage Sir Kim's ambassadorship, to make his position untenable and to have him replaced by somebody more congenial to the leaker," Christopher Meyer, a former British ambassador to the U.S. and close friend of Darroch's, told BBC radio on Tuesday.

There is no formal ban in Britain on filling ambassadorships with political appointees, but it is far from the norm and they tend to be career civil servants. Darroch has spent more than four decades in diplomatic service and was previously a senior British government adviser on terrorism, Russia's annexation of Crimea and thorny Middle East issues from Libya to Iran. About a third of ambassadors appointed by former President Barack Obama were political figures, with the remaining career diplomats, according to the American Foreign Service Association.

Posts filled by Trump so far have appeared to follow a similar trajectory. Robert Wood Johnson, the current U.S. ambassador to Britain, spent more than thirty years as a corporate CEO. He also owns the New York Jets football team.

In his emails, Darroch questioned whether Trump's White House "will ever look competent." He referred to his Iran policy as "incoherent" and "chaotic." Before serving in Washington, Darroch was Britain's ambassador to the European Union, where he sometimes clashed with pro-Brexit politicians such as Trump ally Nigel Farage.

"He should speak to his country, and Prime Minister May, about their failed Brexit negotiation, and not be upset with my criticism of how badly it was handled," Trump tweeted Tuesday, referring to Britain's stalled efforts to leave the 28-nation bloc.

Not long after becoming president, Trump said Farage would make a good ambassador to the U.S., a position Farage has appeared to rule himself out of. "I'm no diplomat," he said Monday. However, Farage did not return a USA TODAY request for comment on whether he would accept the job if it was offered to him. Isabel Oakeshott, the journalist who first reported the leak, is a strong Brexit backer and Farage ally.

Britain's diplomatic community has rallied behind Darroch, who is due to stand down from his role at the start of 2020. "You can’t change an ambassador at the demand of a host country," former Foreign Secretary William Hague told BBC radio.

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

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Brett Bruen, a former U.S. diplomat who served as Director of Global Engagement at the White House, said that ambassadors and their staff in Washington and at U.S. embassies across the globe spend their days trying to squeeze secrets and strategies out of officials, which they then send back with frank, often not glowing assessments.

"If you want to penalize every diplomat who does that you'll cut off relations with pretty much everyone. Even our friends, like the United Kingdom and Israel often say not nice things about us," he said, adding that a diplomat's job is to "wine, dine and forge bonds," then send back an honest analysis. "That's the work of diplomacy."