LONDON — Secret messages revealing how Britain's ambassador criticized President Donald Trump present an acute challenge to the so-called special relationship between Washington and London, former diplomats said Tuesday.

The leaked messages have caused a diplomatic standoff between Washington and London, with Trump saying that "we will no longer deal" with the ambassador, Kim Darroch, and even expanding his attack to target British Prime Minister Theresa May. Trump doubled down Tuesday, calling May "foolish" and the ambassador "wacky" and "very stupid."

Trump's broadsides were triggered by leaked memos by Darroch that revealed the envoy had called the president and his administration "inept," "insecure" and "incompetent."

Kim Darroch presents an award at the White House Correspondents' Dinner pre-party at the British Embassy on April 27, 2018. Brian Stukes / Getty Images file

Even some of those sympathetic to the ambassador have said his job will be impossible if he is blocked by the White House — as Trump promised to do. This prompted warnings that allowing the United States to approve or refuse foreign ambassadors would set a dangerous precedent — not least in more authoritarian parts of the world.

"We should never allow any country to dictate who we send as ambassador," said Charles Parton, who served as a British diplomat for almost four decades. "It would give enormous power to other countries, so you just can't do it."

"If we set this precedent, can you imagine how a country like China would respond?" added Parton, who served as an adviser to the British embassy in Beijing and is currently an adviser to the House of Commons Foreign Affairs Committee.

The ambassador was uninvited from a dinner Trump hosted for the emir of Qatar on Monday night. On Tuesday, Ivanka Trump is due to meet Liam Fox, Britain's international trade secretary. Whether Darroch attends could be telling in terms of how strictly the president's edict will be enforced.

"Is he simply saying, 'You're not going to come to any dinners where I'm present'?" Christopher Meyer, former British ambassador to the U.S., told the BBC on Tuesday. "Or is he going to send an edict out to the entire administration saying, 'Nobody deal with the British ambassador'? We don't know the answer to this."

Jeremy Hunt, Britain's foreign secretary who is competing to replace the outgoing May and head the Conservative Party, warned Trump on Tuesday that if he succeeds, he will keep Darroch as ambassador.

"These comments are disrespectful and wrong to our Prime Minister and my country," Hunt tweeted at Trump, adding, "You said the UK/US alliance was the greatest in history and I agree but allies need to treat each other with respect as @theresa_may has always done with you."

The fallout from this diplomatic standoff has short- and long-term implications.

The principal targets of Trump's ire — the British prime minister and her ambassador — are both due to leave their jobs in a matter of weeks and months respectively. The race to replace May concludes at the end of this month, and Darroch was already set to depart at the end of this year.

Some believe that these changes, combined with the August break in Washington, could take the sting out of the situation. However there are many who think that while Darroch was merely doing his job by communicating his sincerely held opinion back to London, the leak will make his task near impossible.

Robin Renwick, who served as ambassador before Meyer, told the BBC in another interview that he believed the leaker had succeeded in undermining the envoy's position.

"His position has become untenable," he said, adding the ambassador will "have to be moved on."

Britain has real reason to keep its transatlantic ally sweet. The country is set to leave the European Union in October and many within its government see the U.S. as a more vital ally than ever in terms of securing a post-Brexit trade deal.

Fox, the international trade secretary, has already vowed to apologize to Ivanka Trump during the meeting Tuesday, and the previous night, a U.K. government spokesman said the "unfortunate" leak does "not reflect the closeness of, and the esteem in which we hold, the relationship" with the U.S.

The official noises from London have suggested that Darroch will not be replaced. The government spokesman said the ambassador has the "full support" of the government, reiterating the same message given by Foreign Secretary Philip Hammond days earlier.

Long term, one impact might be on the incoming British prime minister and whom they chose as their U.S. ambassador.

The polls suggest former London mayor Boris Johnson is the overwhelming favorite to become the leader of the ruling Conservative Party, and thus prime minister.

The president has praised Johnson, as well as Nigel Farage, the iconoclastic leader of the Brexit Party, who Trump has said "would do a great job" as ambassador to the U.S.