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 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 the headline of the piece in his post: “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.

Retail giants including Asda, Marks & Spencer, Primark and Tesco will mount a new year campaign to warn Rachel Reeves that plans to hike business rates on larger shops will put jobs and stores under threat.

Sky News has learnt that some of Britain’s biggest chains – which also include J Sainsbury, Morrisons and Kingfisher-owned B&Q – have agreed to revive a group called the Retail Jobs Alliance (RJA).

Sources said the RJA, which was established to push for reform of Britain’s archaic business rates regime, is expected to engage with the Treasury in the coming weeks to say that a wave of tax rises and regulatory changes will threaten investment by major retailers in economically deprived areas of the country.

They intend to produce analysis showing many of the stores with so-called rateable values above a new £500,000 threshold are located in areas which rely on retailers for employment opportunities.

The revamped coalition is expected to be launched in January and is likely to include other high street names, according to insiders.

It is said to be coordinating its plans with the British Retail Consortium (BRC), the industry’s leading trade body.

In total, the RJA’s members employ more than a million people across Britain and account for a significant proportion of the stores with rateable values in excess of the proposed threshold.

One source close to the group’s plans said it intended to highlight that the higher business rates multiplier contradicted Labour’s manifesto pledge to “[level] the playing field between high street and online retailers”.

The latest intervention by retail bosses will come after weeks of vocal complaints about the impact of Ms Reeves’s maiden budget on the sector.

Last month, a letter signed by dozens of industry chiefs including from Boots and Next said the budget would pile £7bn of extra costs on to them.

These included a £2.3bn hit from changes to employers’ national insurance, £2.73bn from an increase in the national living wage and a £2bn packaging levy bill.

Retailers have since queued up to warn that consumers will face rising prices when the tax changes come into force in April.

Stuart Machin, the M&S chief executive, and Andrew Higginson, the JD Sports Fashion and BRC chair, have been among those publicly critical of the new measures.

Tesco alone faces having to pay £1bn in extra employer national insurance contributions during this parliament.

This week, ShoeZone, a footwear chain, said it would close 20 shops as a result of poor trading and the increased costs announced in the budget.

The hospitality industry has also highlighted the possibility of price hikes and job losses after the chancellor delivered her statement on 30 October.

In response to the growing business backlash, Ms Reeves told the CBI’s annual conference last month that she was “not coming back with more borrowing or more taxes”.

The RJA was initially put together in 2022 by WPI Strategy, a London-based public affairs firm.

None of the members of the RJA contacted by Sky News this weekend would comment.