The escalation of a diplomatic rift between the U.S. and the U.K. over leaked ambassadorial cables has left former British officials worried over their country’s ability to conduct business in Washington — just when the U.K. was hoping to rely more on the two countries’ “special relationship” as it prepares to leave the European Union.

A British newspaper published leaked diplomatic cables Sunday in which ambassador Kim Darroch described the Trump Administration as “incompetent” and “insecure”.

In response, Trump said Monday he would “no longer deal with” Darroch, who has served as Britain’s ambassador since 2016. On Tuesday, the president lashed out at the U.K. for having “foisted” Darroch on the U.S. and renewed attacks on outgoing British Prime Minister Theresa May over her handling of Brexit.

Some British lawmakers fear the growing dispute could imperil the close ties the two countries have enjoyed since the Second World War — which many had hoped was bolstered during Trump’s state visit to the U.K. in June — and their chances of agreeing on a trade deal after the U.K. leaves the E.U.

“This is such a damaging — potentially damaging — event that I hope the full force of our internal discipline, or even the law, will come down on whoever actually carried out this particular act,” the U.K.’s international trade secretary Liam Fox, the man charged with building up the U.K.’s trading relationships for the post-Brexit era, told the BBC. Fox is currently in the U.S. and said he would apologize for the leak to the president’s daughter, Ivanka Trump, at a scheduled meeting in Washington Tuesday.

“This is going to be a hard period ahead,” says Tom Fletcher, a former British ambassador to Lebanon and former foreign policy adviser to the U.K. government. “It comes at a fragile time in the U.K. — we are vulnerable at the moment, we’re not having the best couple of years.”

In the long run, Fletcher says, the “special relationship” – and U.K.-U.S. cooperation on matters such as security — is resilient enough to survive the blip. But Britain’s next ambassador, due to be appointed in 2020 when Darroch ends his four-year term, “will have to work out how he wants to handle that relationship with the White House — whether to push back or have a more behind-the-scenes role.”

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Fletcher says he was surprised by Trump’s reaction to the leak. “This kind of thing has happened in the past and everyone understood it was part of the game,” Fletcher says. “But in the past we haven’t had such a thin-skinned person in the White House. It’s really unfortunate that escalated so fast.”

Other former diplomats have defended the content of Darroch’s memos. “He has done nothing wrong,” Christopher Meyer, who served as British ambassador to the U.S. from 1997 to 2003, told Sky News, adding that giving U.K officials a “very candid assessment” of foreign governments is “what an ambassador is paid to do.”

But others warn the leak could prevent British diplomats from doing that job.

“It’s embarrassing that we all know Britain’s ambassador believes the US administration is ‘dysfunctional,’ ‘unpredictable,’ ‘faction-riven’ and ‘inept,'” Peter Ricketts, a former U.K. ambassador to France, wrote in an op-ed published in The Guardian Monday. “But the most toxic aspect of the latest leak is that it undermines trust in that confidentiality.”

“If or when Britain does finally leave the European Union, negotiating a new basis for relations with our former partners and rethinking national strategy will put unprecedented demands [on officials]. It will be essential that ministers get the best advice from their civil servants, and that decisions are based on pooling all the key information.”

Peter Westmacott, Darroch’s predecessor as ambassador to the U.S. from 2012 to 2016, wrote in The Telegraph newspaper that the leak put Darroch in “an embarrassing position” but said the real damage was to the diplomats’ ability “to say what they really think to ministers back home” and the risk that offended host country would make it “much harder” for diplomats to do their jobs if they are deemed “unsympathetic.”

Westmacott suggested the leak may have been intended to pressure May’s successor as Prime Minister — likely to be the Conservative lawmaker Boris Johnson — into choosing “someone already regarded as a ‘friend’ by the Trump administration” and “less ready to speak truth to power” as Darroch’s successor after his four-year stint ends in 2020.

Fletcher says the leak was not necessarily so tactical. “In my experience working in government, it’s almost always a [mistake] rather than a conspiracy,” he says. “In any case, that appointment will be one of the first things on the next prime minister’s desk. We’ll see if President Trump claims that as a victory when it happens.”