Lord Mandelson is expected to be named as the UK's next ambassador to the Unites States.
The Labour grandee will replace Dame Karen Price, whose term in Washington DC will end in early 2025.
He served in multiple ministerial roles under Tony Blair and Gordon Brown before he was elected to the House of Lords, and was considered to be a frontrunner for the position.
Lord Mandelson was also seen as a key advisor to the prime minister Sir Keir Starmer in the run-up to the 2024 general election where he hosted a podcast for The Times and served as a trustee of the Design Museum and chancellor of Manchester Metropolitan University.
Sir David Manning, who served as the UK's ambassador to the US between 2003 and 2007, told BBC Radio 4's The World Tonight programme that Lord Mandelson was a "very articulate, highly intelligent, extremely experienced operator."
His appointment comes ahead of the inauguration of President-elect Donald Trump on January 20.