The co-manager for Donald Trump’s presidential election campaign has called Lord Mandelson an “absolute moron” – as the peer was officially announced as the next UK ambassador to the US.

The Labour grandee, who served in Sir Tony Blair and Gordon Brown’s cabinets, will become the UK’s top diplomat in Washington as president-elect Donald Trump returns to the White House in January.

In a post on X, Chris LaCivita linked to an article in The Daily Telegraph which reported on Lord Mandelson describing Mr Trump as “a danger to the world”.

The peer also described Mr Trump as “little short of a white nationalist and racist” during an interview with an Italian journalist in 2019, according to the newspaper.

In the post on X, Mr LaCivita said: “This UK govt is special replace a professional universally respected Ambo with an absolute moron – he should stay home! SAD!”

He added: “Mandelson described Trump as a danger to the world and ‘little short of a white nationalist’.”

It comes as Sir Keir Starmer confirmed the appointment of Lord Mandelson to the post.

The 71-year-old said: “We face challenges in Britain but also big opportunities and it will be a privilege to work with the government to land those opportunities, both for our economy and our nation’s security, and to advance our historic alliance with the United States.”

The prime minister said: “The United States is one of our most important allies and as we move into a new chapter in our friendship, Peter will bring unrivalled experience to the role and take our partnership from strength to strength.”

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Sir Keir also thanked outgoing US ambassador Dame Karen Pierce, who will leave the post at the beginning of next year.

“I would also like to thank Dame Karen Pierce for her invaluable service for the last four years, and in particular the wisdom and steadfast support she has given me personally since July,” he said.

“She made history as the first woman to serve as UK ambassador to the US and she has been an outstanding representative of our country abroad. I wish her all the very best in future.”

Lord Mandelson was one of the key architects of New Labour and helped the party return to power in the 1990s.

He served as Sir Tony’s trade secretary and Northern Ireland secretary before standing down as an MP in 2004 to become a European Commissioner.

After Mr Brown awarded him a peerage in 2008 Lord Mandelson returned to government as business secretary.