Starmer, who is struggling to contain the fallout over his decision to name Mandelson to the post, admitted he had been wrong to appoint the 72-year-old.
Mandelson was already a known associate of late US sex offender Jeffrey Epstein and had twice had to resign from earlier Labour government posts.
Addressing parliament again about the deepening political row, Starmer said: “At the heart of this, there is also a judgment I made that was wrong. I should not have appointed Peter Mandelson.”
The scandal has threatened to bring down the prime minister, who faced fresh calls to quit last week after it was revealed that Mandelson had failed security checks.
Already unpopular with the public and some Labour MPs, Starmer has insisted he and other ministers had not been told until last week that the Mandelson’s security approval had been declined.
“It beggars belief that throughout the whole timeline of events, officials in the foreign office saw fit to withhold this information from the most senior ministers in our system, in government,” he told MPs.
The information about Mandelson’s failed security vetting had been withheld from him, he said.
“I was not provided with information I should have been provided with. Had I been provided, I wouldn’t have made the decision. It was a deliberate decision. It wasn’t negligence. It was a deliberate decision not to tell me.”
Two lawmakers – one from the left-wing Your Party and another from the far-right Reform UK, were removed from the session for accusing Starmer of lying over the issue and refusing to withdraw their statements.
“He is gaslighting the nation. So let’s call this out for what it is. The prime minister is a bare-faced liar,” said left-winger Zarah Sultana before being ordered out by the speaker.
‘Unconventional’
Last Thursday, Starmer sacked the foreign office’s top civil servant, Olly Robbins, telling MPs he had also now set in motion a review of the security vetting process.
But former civil servants have accused Starmer of scapegoating Robbins, who will give his own account to a parliamentary watchdog committee on Tuesday.
Lawmakers will also hold an emergency debate in parliament after the leader of the main opposition Conservative Party, Kemi Badenoch, said there remained “serious questions about what he (Starmer) knew and when”.
Opposition leaders have called for Starmer to step down, with accusations ranging from incompetence to the wilful misleading of parliamentarians and the public.
“We still do not know exactly why Peter Mandelson failed that vetting,” Badenoch told parliament.
She and Liberal Democrat leader Ed Davey again called for Starmer to resign.
Davey said the prime minister had made “a catastrophic error of judgment, and now that it’s blown up in his face, the only decent thing to do is to take responsibility”.
Senior ministers have however so far rallied around Starmer.
“A judgment was made that the Trump administration was an unconventional administration and an unconventional ambassador could do a job for the United Kingdom,” Scotland Secretary Douglas Alexander said Monday.
‘He has to go’
Other ministers have argued that Starmer should remain in power amid the global tumult sparked by the Middle East war.
But polls suggest Starmer is one of Britain’s most unpopular prime ministers ever.
If the centre-left Labour leader knew about the failed vetting “then he has to go, he has to resign”, retired dentist Andrews Connell, 59, told AFP.
But retiree Duncan Moss, 67, said he would be “very worried if Starmer was to leave … I think he’s doing a very good job.”
Starmer sacked Mandelson in September 2025, seven months after he took up the post, after new details emerged about the depth of the ex-envoy’s ties to Epstein, who died in a US prison in 2019 while facing sex-trafficking charges.
UK police are investigating allegations of misconduct in office by Mandelson when he was a Labour minister more than 15 years ago.
He was arrested and released in February. and has not been charged. Mandelson denies criminal wrongdoing.
Starmer and the Labour party are also bracing for a potentially chastening set of local elections next month, including in the devolved Scottish and Welsh parliaments.