Lord Peter Mandelson, the next UK ambassador to the US, has praised Donald Trump and said his will be “one of the most consequential” presidencies in modern times.

In an article for Fox News, he hailed the US President-elect’s “straight talking and deal-making instincts” and his “great skill as a political campaigner”.

The comments are a stark contrast to comments in 2019, when he called Trump a “danger to the world” and “little short of a white nationalist”. A top Trump campaign advisor later branded Lord Mandelson “an absolute moron”.

Known as a key figure in both Tony Blair and Gordon Brown’s governments, Lord Mandelson will take office as the UK’s envoy to Washington DC in early 2025.

In his article, Lord Mandelson argued Britain’s “Atlantic alliance” cannot “enjoy its glorious past” but must instead “adapt to new threats and technological challenges”.

He wrote: “America’s allies need to hear the message the American people have sent and calibrate their partnerships in order to work with and alongside the US to stand up for the joint interests that unite us.

“Trump’s great skill as a political campaigner is to tap into sections of an electorate that feel unheard.”

The Republican candidate’s “resounding victory” should be viewed by allies as a “significant opportunity to bring entrepreneurial thinking and urgency to resolving foreign policy that needs real-world solutions,” he added.

“The Trump administration will rightly put the needs of Americans first,” Lord Mandelson said.

“But in its closest allies, it will find old partners open to new ways of dealing with the current world – to protect, enrich and build opportunities for Americans and our own citizens alike.”

The International Monetary Fund has warned Trump’s threatened wave of tariffs could disrupt economies across the world and worsen diplomatic tensions.

Tariffs are a central part of the president-elect’s economic vision – he sees them as a way of growing the US economy, protecting jobs and raising tax revenue, and has threatened to impose them on China, Canada and Mexico on day one of his presidency next week.

During his election campaign, Trump pledged to introduce a blanket charge on all foreign goods to protect domestic industries.

Analysis by the University of Sussex’s Centre for Inclusive Trade Policy (CITP) found this could result in a £22bn hit to the UK economy.

UK exports to the world could fall by more than 2.6% because of reduced trade with the US and knock-on effects globally, the report found.

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.