UK leader Keir Starmer insisted on Monday that he would never have appointed Peter Mandelson as his ambassador in Washington if he had known the full extent of the diplomat's relationship with sex offender Jeffrey Epstein.

The comments, Starmer's first since he sacked Mandelson last week, come as the prime minister faces growing questions about his judgement, including from his own Labour MPs.

Starmer, whose popularity has tanked since taking office in July 2024, has endured several days of negative front pages in UK newspapers, with some even suggesting he is on borrowed time.

The prime minister backed Mandelson last Wednesday, only to fire him the following day after emails showed that Mandelson had encouraged Epstein to "fight for early release" shortly before he was sentenced to 18 months in prison in 2008 for procuring a child for prostitution.

"Had I known then what I know now, I'd have never appointed him," said Starmer, who told broadcasters that Mandelson had been properly vetted before his appointment.

Some Labour MPs have questioned why Starmer appointed the former government minister when it was public knowledge that he had remained friendly with Epstein long after his conviction.

The government has also faced questions over why Starmer said he had "confidence" in Mandelson last week when the then-ambassador had already acknowledged that "very embarrassing" messages were about to be released.

The sacking came less than a week after Angela Rayner was forced to resign as deputy prime minister after she admitted to underpaying on a property tax.

The high-profile departures came just after Starmer launched a reboot of his government in a bid to start clawing back support from the hard-right Reform UK party, led by Nigel Farage. (AFP)