Britain scrambled on Monday to stem damage to its relations with Washington and find the leaker of cables in which the UK ambassador called US President Donald Trump "inept".
The confidential telegrams from ambassador Kim Darroch created a political firestorm in London after their publication in the Mail on Sunday newspaper.
One of them called the Trump administration "uniquely disfunctional" while another characterised the US leader as "incompetent" and "insecure".
Their release came just a month after Trump's state visit to Britain during which he visibly enjoyed himself while dining with Queen Elizabeth II and touring Buckingham Palace with his family.
They also threatened to complicate London's efforts to strike a new trade deal with Washington that could help mitigate potential damage from Britain's withdrawal from the European Union.
Trump fired back by saying that people in his administration were "not big fans" of London's man in Washington.
"The ambassador has not served the UK well, I can tell you that," Trump told reporters on Sunday.
"We are not big fans of that man and he has not served the UK well. So I can understand it, and I can say things about him but I won't bother."
- 'Very serious consequences' -
UK officials took turns defending Darroch as a professional who was carrying out his duties by providing "frank" assessments of the latest developments in the White House.
"We need to find out how it happened, not least to give confidence to our teams all over the world that they can continue to give us their frank assessments," Foreign Secretary Jeremy Hunt said on Monday.
"I hope we get to the bottom of it, and of course there will be very serious consequences if and when we find out who was responsible."
The Daily Telegraph newspaper said such memos are seen by up to 100 people working in the Foreign Office and other government departments.
"But it would require a single official or minister to have access to the whole cache, inevitably casting the spotlight on senior ministers," it wrote.
The immediate suspicion of the London papers fell on Brexit-backing players in a power struggle within Britain's governing Conservative Party.
British politics are in for a major revamp once Prime Minister Theresa May ends her three-year spell in power later this month.
Brexiteer former foreign minister Boris Johnson is the favourite to replace her in a leadership contest against Hunt.
The winner will get to appoint his own US envoy by the time Darroch's term expires at the end of the year.
But commentators note the official widely tipped for the Washington job was less enthusiastic about Brexit than Johnson or some members of his team.
"There have been reports that Mark Sedwill, the current cabinet secretary, is being lined up for the position," The Guardian's diplomatic editor Patrick Wintour wrote.
But Trump "may be suspicious of Sedwill, and would prefer a true Brexiter".
Wintour wrote that Sedwill "may not fill that bill, but Nigel Farage -- who pretty openly campaigned for the job before Darroch's appointment in 2016 -- does".
Farage is an anti-EU populist whose new Brexit Party won nearly a third of the vote in European elections in May, and has met Trump several times.
Farage on Monday called Darroch's memos "pretty irresponsible" but played down interest in the Washington post.
"I'm not a diplomat, and I think that's quite an understatement," he told BBC radio.
But Farage added that he "could be very useful" in forging a better relationship "with an administration that contains friends of mine".