Aides close to US president Donald Trump yesterday called for Sir Kim Darroch's sacking as UK ambassador to Washington after he branded the President's administration "inept" and "incompetent".
Darroch's career was hanging in the balance after sensitive diplomatic cables he circulated over the past two years were leaked and made public in a Sunday newspaper.
As the Foreign Office launched an investigation, Jeremy Hunt, the Foreign Secretary, gave his full backing to Trump while refusing to support Darroch.
A self-confessed Europhile who spent years in Brussels, Darroch may have been targeted because he was too pro-Europe and not the right person to promote post-Brexit trade relations with the US.
White House officials suggested a new prime minister, either Hunt or Boris Johnson, would restore relations once the furore died down.
Yesterday allies of Mr Trump said Darroch could "no longer work with the president", and he would be unable to "serve the UK effectively".
According to the cables Darroch sent to London, the White House had become "uniquely dysfunctional", dogged by internal "knife fights".
"We don't really believe this administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction riven; less diplomatically clumsy and inept," Darroch wrote.
Mystery surrounds who leaked the memos and why. According to a source close to Isabel Oakeshott, the journalist who obtained the memos, they were passed to her because she is a well-known Brexiteer.
The source said: "There are pro-Brexit officials who feel like pariahs and believe the civil service is institutionally pro-Remain. There is certainly a view that Sir Kim, with his long background in Brussels and as a former adviser to Tony Blair on Europe, had not made the most of his time in Washington to explore post-Brexit opportunities."
One MP said: "Maybe somebody is trying to give Sir Kim a push, but I wish they weren't — it only damages our decision making. This is coming from some Brexit faction — to damage Darroch or to possibly influence Boris Johnson to pick a more 'Brexit-friendly' US ambassador."
A senior Foreign Office source said: "It looks like mischief making and I wouldn't be surprised if a Cabinet minister or a senior minister got these and they could easily have passed them on.
Exposing ambassador's private messages damages everyone
COMMENT by Peter Westmacott
There are two sides to the role of an ambassador, one public and one private. In public, you represent your country through the media, speeches, and events with the aim of making friends and influencing people. In private, you conduct delicate negotiations; and you report back, frankly, to your government on what is happening while offering honest advice on how best to pursue your nation's interests.
The private messages from Sir Kim Darroch, my successor as British ambassador in Washington, made public over the weekend, clearly fell into the second category.
Clearly, the leaking of this series of carefully-selected documents written over more than two years was no accident. A proper leak investigation should shed light on how it happened, and why.
I have no idea who thinks they stand to gain by placing Sir Kim in such an embarrassing position — he is due to leave at the end of his four-year stint in a few months' time.
Perhaps there are people at the heart of government who think this will persuade the incoming Prime Minister to appoint someone less ready to speak truth to power than Sir Kim has been; or someone already regarded as a "friend" by the Trump administration.
Whatever the explanation, leaking this material was wrong, for at least three reasons.
First, it was against the code of confidentiality that applies to all who work on sensitive matters within government — and may have been against the Official Secrets Act.
Second, giving sensitive papers of this kind to the media is the biggest possible disincentive to ambassadors to say what they really think to ministers back home.
Third, it carries the risk of causing offence to the host government, which in turn can undermine key relationships.
Finally, a word of caution about choosing "friends" of host governments as ambassadors. Elections can change administrations, especially when there is the kind of polarisation that currently exists between Democrats and Republicans in the US, an ambassador deemed to be too close to one side could easily become persona non grata with the other.
There are good reasons why Britain has a long tradition of apolitical civil and diplomatic services.
• Sir Peter John Westmacott was British Ambassador to the United States from 2012 to 2016