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

This followed a 2018 interview with the Evening Standard where he described Trump as “a bully”.

But he told Fox News, external: “I made those remarks six years ago in 2019, led rather along this by an Italian journalist… it was a time in Britain by the way with very fraught politics and there was high emotion about many things in Britain at that time.

“I consider my remarks about President Trump as ill-judged and wrong.

“And I think that time and attitudes towards the president has changed since then.”

Acknowledging Trump’s “extraordinary” second mandate, Lord Mandelson said Trump had won “fresh respect” from him and said he had heard nothing from the White House that suggested there was going to be any difficulty about his appointment.

“I think that President Trump will look at my credentials and consider what’s best for the relationship going forward, ” he said.

“The president is a nice person, is a fair-minded person, and that’s why I feel quite confident that, when as I say he does look at my credentials he’ll think ‘right, I think this guy could work well for both our countries and our special relationship’.”

He added Trump could be “one of the most consequential” US presidents.

The government is much less concerned than previously about the possibility that Lord Mandelson’s appointment could be blocked by Trump.

Some of the president’s allies had suggested that the president could take the extraordinary step of blocking him from taking up the role.

Trump’s co-presidential campaign manager Chris LaCivita last month warned Lord Mandelson to “stay home” and called him “an absolute moron” over his past comments on Trump.

But Lord Mandelson is understood to have now received his “agrément” from the US government – a formal step confirming their willingness to accept him as a diplomat in their country.

The final hurdle will come when Lord Mandelson presents his “letter of credence” to Trump.

It is understood that he is likely to fly to Washington DC to carry out the final formalities next week.

A Foreign Office spokesperson said: “Lord Mandelson is HMA-designate and the formal diplomatic process is ongoing.”

The US State Department has also been contacted for comment.

Mandelson’s comments go further than others in government in recanting previous criticism of Trump.

Others in government such as the Foreign Secretary David Lammy have typically sought to deflect questions about their attacks on Trump by pointing out that many senior Republican politicians have previously criticised him too.

On Sunday, Prime Minister Sir Keir Starmer had his first phone call with Trump since the inauguration, but its is thought Lord Mandelson’s appointment was not discussed.

Lord Mandelson is a well-known figure 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 was appointed by Sir Keir in December and will replace outgoing ambassador Dame Karen Pierce, the first woman to serve as UK ambassador to the US.