Lord Mandelson will replace Dame Karen Pierce, whose term in Washington DC is due to end around the time Trump enters the White House on Monday.

In his article, he also echoed strong US criticism of both China and Iran.

Lord Mandelson – who once chaired a public body supporting UK-China relations – said the current Chinese government was “more aggressive abroad and controlling at home” in a way that “directly challenges Western governments and our values”.

He described Iran as a “thoroughly malign force” in the Middle East and added: “We must not ever allow it to become nuclear armed”.

Ambassadors do their work behind closed doors, but Lord Mandelson is engaging in some public diplomacy days before Trump takes office.

His article is also a sign that the man once dubbed the “Prince of Darkness” for his work as a Labour spin doctor is going to do some of his diplomacy in the light.

And that is not always without risk.

The public record of Lord Mandelson’s criticism of Trump, like other senior Labour figures, could cause diplomatic trouble for the government.

In a recently unearthed interview with an Italian journalist in 2019, Lord Mandelson described Trump as a “reckless and a danger to the world”.

In a 2018 interview with the Evening Standard, he also called Trump “a bully”.

In response to the comments, Trump’s co-campaign manager Chris LaCivita said Lord Mandelson “should stay home” rather than serve as ambassador.

Mr LaCivita, who currently who has no official role in the next White House, criticised the British government’s decision saying it was replacing a “professional universally respected ambo [ambassador] with an absolute moron”.

Since being touted as a potential candidate for the US ambassador role, considered the most prestigious UK diplomatic post, Lord Mandelson has significantly softened his language.