He described the Labour government in a social media post as “special” — saying it had “replaced a professional universally respected Ambo with an absolute moron — he should stay home! SAD!”

However, No 10 hit back strongly, describing Lord Mandelson as a “significant asset” to diplomacy.

A spokesperson for Sir Keir said: “His appointment shows just how seriously we’re taking this relationship.

“He has got extensive foreign policy and economic policy expertise, particularly in the crucial issues of trade, business links.

“He’s got experience at the highest levels of government and it will be a significant asset in the UK’s relationship with the United States.”

The Labour peer will take up his post in early January and will be tasked with forging close links with the new Trump administration.

Relations between Trump and the Labour Party hit rock bottom during the US election campaign.

Trump’s team accused Labour of “blatant foreign interference”, filing a complaint with the Federal Election Commission (FEC).

They alleged that Labour Party activists were campaigning directly for Kamala Harris and her running mate Tim Walz.