British Prime Minister Keir Starmer backed the UK ambassador to Washington Peter Mandelson Wednesday after details emerged of the diplomat's friendship with disgraced sex offender Jeffrey Epstein.

Mandelson called the late Epstein his "best pal" and an "intelligent, sharp-witted man" in a 2003 letter, released to the public just a week before US President Donald Trump is due to pay a state visit to Britain.

Challenged in parliament about his judgement in appointing the 71-year-old grandee of the centre-left Labour party to the key diplomatic post, Starmer insisted that "due process was followed".

He described Epstein as a "despicable criminal" who "destroyed the lives of so many women and girls".

But he added: "The ambassador has repeatedly expressed his deep regret for his association with him. He is right to do so. I have confidence in him."

The letter was one of many included in a book compiled to mark the now notorious financier's 50th birthday.

The contents were published by a US congressional panel investigating Epstein's sex crimes case.

- 'Prince of Darkness' -

Mandelson, an influential former Labour minister and spin doctor, said it was "very embarrassing" to see the letter published, in comments to The Sun daily's "Harry Cole Saves the West" YouTube channel.

"I regret very, very deeply indeed carrying on that association with him for far longer than I should have done," the ambassador said.

He said he had never witnessed any criminal behaviour, but added he also felt a deep sense of sympathy for the women "who suffered as a result of (Epstein's) behaviour and his illegal criminal activities".

Mandelson conceded that further embarrassing correspondence between himself and Epstein will come out, meaning Starmer will likely face further tricky questions about the appointment.

"I have no doubt at all that there's a lot of traffic, correspondence exchanges between us, absolutely. And we know those are going to surface," said Mandelson.

"We know they're going to be very embarrassing, and they know that I'm going to profoundly regret ever having met him and been introduced to him in the first place."

Mandelson, dubbed the "Prince of Darkness" during his years as a political spinner, was twice forced to resign from Tony Blair's Labour government in the late 1990s and early 2000s over allegations of misconduct.

Starmer lost Angela Rayner as his deputy prime minister last week after she resigned for underpaying a property tax two days after the PM gave her his backing.