Peter Mandelson is set to become Britain’s next ambassador to the US,, the first time a politician has been appointed to the role for almost half a century.

Keir Starmer is about to announce that Lord Mandelson, a former Labour minister and European commissioner for trade, has been given the role. The Guardian understands he will take over as Donald Trump begins his second term as president.

The news comes as the UK prepares for challenging changes in trade relations with the US under the president-elect. The prime minister’s chief of staff, Morgan McSweeeny, travelled to Washington for talks with Trump’s incoming White House team earlier this month.

The prime minister believes that Mandelson, a former Labour MP who now sits in the House of Lords, has the trade expertise and networking abilities to bolster the UK’s interests during this “delicate period” for its relationship with the US, according to the Times.

He served as business secretary under Gordon Brown, was made president of the Board of Trade in 1998 and was the European Commissioner for Trade in 2004.

A government source told the BBC: “The fact the prime minister has chosen to make a political appointment and sent Lord Mandelson to Washington shows just how importantly we see our relationship with the Trump administration.

“We’re sending someone close to the prime minister with unrivalled political and policy experience, particularly on the crucial issue of trade. He’s the ideal candidate to represent the UK’s economic and security interests in the USA.”

Trump has promised to issue universal tariffs of up to 20%, which will hit all goods imported into the US from overseas and is likely to drive up the cost of these goods for American consumers.

In November, Stephen Moore, one of Trump’s senior economic advisers, said that if the UK moved towards the US model of “economic freedom” there would be more “willingness” by the incoming administration to agree a trade deal between the two countries.

Speaking to BBC’s Today programme, Moore said: “The UK really has to choose between the Europe economic model of more socialism and the US model, which is more based on a free enterprise system. I think the UK is kind of caught in the middle of these two forms of an economic model.

“I believe that Britain would be better off moving towards more of the American model of economic freedom.”

But Starmer recently rejected warnings that Trump could pressure the UK to, in effect, pick sides between the US and the EU. “Against the backdrop of these dangerous times, the idea that we must choose between our allies, that somehow we’re with either America or Europe, is plain wrong,” he said.

“I reject it utterly. [Clement] Attlee did not choose between allies. [Winston] Churchill did not choose. The national interest demands that we work with both.”

He added that he would “never turn away” from the UK’s special relationship with the US. “This is not about sentimentality,” he said. “It is about hard-headed realism. Time and again the best hope for the world and the surest way to serve our mutual national interest has come from our two nations working together. It still does.”

Mandelson, a Labour veteran who worked as the party’s director of communications in the 1980s, previously told the Times’s How to Win an Election podcast that Britain must find a path between the US and the EU if Donald Trump goes ahead with his pledge to introduce blanket tariffs on goods.

“We have got to navigate our way through this and have, I’m afraid, the best of both worlds,” he said. “We have got to find a way to have our cake and eat it.”

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