A senior aide to Donald Trump has called Peter Mandelson "an absolute moron" after the Labour peer was named the UK's next ambassador to the US.
Lord Mandelson, who served in the cabinets of Sir Tony Blair and Gordon Brown, was called the UK's next top diplomat in Washington by Keir Starmer on Friday.
But Chris LaCivita, the co-chairman of Mr Trump's successful election campaign, highlighted an interview Lord Mandelson gave in which he called the president-elect "little short of a white nationalist and racist".
Lord Mandelson also described Mr Trump as "a danger to the world" in the interview with an Italian outlet.
Mr LaCivita said: "This UK govt is special replace a professional universally respected Ambo with an absolute moron – he should stay home! SAD!"
Announcing the appointment earlier, Keir Starmer said: "I am delighted to appoint Lord Mandelson to be the next British Ambassador to the United States of America.
"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."
He also thanked outgoing ambassor Karen Pierce "for her invaluable service for the last four years".
Lord Mandelson said: "It is a great honour to serve the country in this way.
"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.”
In November, the former MP for Hartlepool previously said he would be "very interested" in giving advice on trade to whoever got the job.
It comes as Keir Starmer seeks to avoid damaging trade tariffs that Trump has threatened to impose.
Lord Peter Mandelson praises Keir Starmer
Author Robert Harris told LBC Lord Mandelson will "fit in well in Washington DC" and that it's an "appointment that makes a lot of sense".
On Thursday evening, author Robert Harris told Iain Dale that Lord Mandelson has "star quality" and will fit in well in Washington DC.
"He's a big figure, whether you like it like him or not.
"I mean, he's a big player and has had a big impact on British politics, therefore he'll speak with some authority.
added: "I think that it was quite significant that actually, Nigel Farage, of all people, said that Peter Mandelson would make a good ambassador, and he has Trump's ear, so that struck me almost as a kind of invitation."
He also described him as an "expert on trade world trade".
"That was his brief when he was a European Commissioner.
"I can't think there's a British politician who knows more about that issue."