James Roscoe played a key role in President Donald Trump's state visit to the UK and in King Charles III's recent visit to the US. Photo: shutterstock

Foreign Office officials declined to give any explanation for why Roscoe had, as they put it, "left his post".

Until his sudden departure, Roscoe held one of the most senior, high-profile roles in the British diplomatic service, as second-in-command at the British Embassy in Washington.

He had also stood in for Lord Peter Mandelson for several months after he was sacked last year.

Roscoe was one of those tipped to take over the role, which ultimately went to another official, Sir Christian Turner.

He played a key role in President Donald Trump's state visit to the UK and in King Charles III's recent visit to the US.

The British Embassy declined to comment further about his departure.

Roscoe could not be reached for comment.

Prior to his move to Washington, he was the UK ambassador to the United Nations and previously served as communications chief to the late Queen Elizabeth II.

He had also been chief press officer in Downing Street for prime ministers Tony Blair and Gordon Brown.

Roscoe's earlier diplomatic postings included the UN, Sierra Leone and Iraq.