James Roscoe, the Deputy British ambassador to the US, has been mysteriously sacked.
Embassy staff were informed of his dismissal in a one-line email with no other explanation, it was revealed last night.
It's believed the exact reason for his departure will be explained on Wednesday to a gathering of diplomats.
A Foreign Office spokesman, meanwhile, simply said: "James Roscoe has left his post."
Mr Roscoe had been facing questions as part of a probe into leaks of top-secret discussions at a meeting of the National Security Council, The Times reported.
He had previously served as acting ambassador to the US after Lord Mandelson's exit - before the appointment of Sir Christian Turner.
Sir Christian has attempted to revive the "special relationship" amid tensions over Britain's early inaction as the Iran war began.
But the career diplomat and UK ambassador to the US came under fire in April for his leaked remarks about America's "real" special relationship.
"'Special relationship' is a phrase I try not to utter because it's quite nostalgic, it's quite backwards-looking, and it has a lot of sort of baggage about it," he told a group of UK sixth form students visiting the US.
James Roscoe served as acting ambassador to the US after Lord Mandelson resigned
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"I think there is probably one country that has a special relationship with the United States... and that is probably Israel," he added.
Sir Christian made the leaked remarks in February, just weeks after replacing Lord Mandelson as ambassador.
Lord Mandelson was sacked over his connections to convicted paedophile Jeffrey Epstein.
The scandal around Lord Mandelson's vetting process also saw the contentious sacking of Sir Olly Robbins, who said he faced an "atmosphere of constant pressure" to appoint the former Business Secretary to the top post.
THE 'SPECIAL RELATIONSHIP' - READ MORE:
Sir Christian Turner, the British ambassador to the US, said the real 'special relationship' was between America and Israel | GETTYThe Government has released some redacted "Mandelson files" after the Cabinet Office was ordered to disclose data related to Lord Mandelson's appointment.
But the full files have not yet been released due to the ongoing investigation into the former ambassador.
The King's state visit to the US has eased tensions between the two countries - just as Sir Keir Starmer faces down a Labour civil war thanks to his party's wipeout at the local elections.
And when asked if the Prime Minister should quit, Donald Trump said: "I don't say [that]. I think he's a nice man, actually.
Lord Mandelson was sacked from his role over his connections to convicted paedophile Jeffrey Epstein
|GETTY
Though the President added: "But I mean, I didn't like what he said... ‘We're going to send ships as soon as you're finished with the war'.
"He's in a lot of trouble."
Mr Trump has long blasted the PM for his energy and immigration policies - and has told GB News Sir Keir must "open up the North Sea immediately".
Mr Roscoe previously served as chief press officer in No10 for Sir Tony Blair and then Gordon Brown, from 2006 to 2009.
He also served as Britain's ambassador to the UN from 2019, and had been described as a "consummate insider for the royals" and an "unflappable" diplomat.
Mr Roscoe is also married to Clemency Burton-Hill, a violinist and author who he met in Sierra Leone, while she was on assignment for a newspaper.