A top campaign adviser to US President-elect Donald Trump has called the incoming UK ambassador to the US, Lord Peter Mandelson, “an absolute moron”.

In a post on social media, Chris LaCivita said Lord Mandelson “should stay home”.

Mr LaCivita, who was a co-campaign manager for Trump’s presidential election bid, criticised the British government’s decision saying it was replacing a “professional universally respected ambo [ambassador] with an absolute moron”.

Lord Mandelson is one of the best-known figures in British politics, having served in multiple ministerial roles under Tony Blair and Gordon Brown before taking up a life peerage in the Lords.

He called his appointment as the UK’s next ambassador to the US as “a great honour”.

As first reported in The Times, Lord Mandelson will replace Dame Karen Pierce, whose term in Washington DC is due to end as Trump enters the White House in early 2025.

Dubbed the “Prince of Darkness” during his years as New Labour’s spin doctor, the 71-year-old will now be the key link between the prime minister and Trump’s incoming administration during a crucial time for US-UK diplomacy.

Like other senior Labour figures, Lord Mandelson has a record of criticising Donald Trump, once describing him as “little short of a white nationalist and racist”.

Those comments were the focus of Mr LaCivita’s criticism of Lord Mandelson, as he said in his post on X that the incoming ambassador “described Trump as a danger to the world and ‘little short of a white nationalist'”.

Mr LaCivita, a veteran of Republican politics with decades of experience, was a senior adviser to Trump’s 2024 election campaign but currently has no official role in the administration.

In a statement following his appointment, Lord Mandelson 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.”

Prime Minister Sir Keir Starmer said he was “delighted” to appoint Lord Mandelson.

“The United States is one of our most important allies and as we move into a new chapter in our friendship,” he said in a statement.

“Peter will bring unrivalled experience to the role and take our partnership from strength to strength.”

Sir Keir also thanked Dame Karen for “her invaluable service for the last four years, and in particular the wisdom and steadfast support she has given me personally since July”.

UK ambassadors are normally career diplomats or civil servants, but Downing Street said choosing a leading Labour politician “shows just how importantly we see our relationship with the Trump administration”.

It comes as senior Conservative MP Sir Iain Duncan Smith challenged the decision.

He called for an investigation to scrutinise Lord Mandelson’s appointment, his background and “whether or not this is reliable or anyway likely to cause offence in the United States”.

“He’s not a diplomatic appointee, he’s a political appointee and political appointees often carry baggage, particularly if they’ve been out of parliament and out of government for some time,” Sir Iain added.

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”.

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

In November he made a pitch on the BBC’s Sunday with Laura Kuenssberg programme to create “a new relationship rather than a special one” with the US.

He also told News Agents podcast it is “absolutely essential that we establish a relationship with President Trump that enables us not only to understand and interpret what he’s doing but to influence it”.

He added that the Labour government should try to “reconnect” with Trump’s ally and tech multi-billionaire Elon Musk.

Musk, who has been critical of Sir Keir’s government, has been appointed head of new advisory team the Department of Government Efficiency (Doge), which is not an official government department.