UK Prime Minister Keir Starmer, battling for his political future, apologised on Thursday to victims of Jeffrey Epstein for appointing disgraced Peter Mandelson as US ambassador.

Starmer issued the wide-ranging apology after being dogged for days by the decision, following fresh allegations about Mandelson's cosy ties to late sex offender Epstein which emerged in newly-released files last Friday.

The latest crisis to hit Starmer's struggling government has left many doubting his judgement and some – including within his ruling centre-left Labour party – questioning if he can remain prime minister.

"I am sorry," an impassioned Starmer said, addressing his apology to Epstein victims who "have lived with trauma that most of us can barely comprehend" and "have seen accountability delayed and too often denied to them".

"Sorry for what was done to you, sorry that so many people with power failed you, sorry for having believed Mandelson's lies and appointed him," he added, ahead of delivering a speech on defending British values.

He also vowed to remain Britain's prime minister.

"I intend to go on doing that vital work for our country because I believe it is the absolute focus and of first importance for this government," he told reporters.

Starmer fired former UK minister and EU trade commissioner Mandelson last September after only seven months as ambassador in Washington, following an earlier release of files about Epstein.

The ex-envoy was one of numerous prominent figures again embarrassed by last week's latest revelations of ties to the late US financier, who died in jail in 2019 by suicide facing charges of alleged sex trafficking.

Email exchanges between them showed a warm, intimate friendship, financial dealings, private photos as well as evidence that Mandelson passed confidential and potentially market-sensitive information to Epstein nearly two decades ago.

Starmer has said Mandelson repeatedly lied to secure the Washington job but insists he had not previously known about the "depth and extent" of his friendship with Epstein.

However, the prime minister confirmed on Wednesday he was aware ties had endured despite the disgraced financier's 2008 US conviction for soliciting a minor. (AFP)