Starmer, who won the largest majority in modern history for Labour at a general election in 2024, is facing fresh calls to step down over the Mandelson affair, just three weeks before his party is expected to suffer big losses in local elections in England and regional votes in Scotland and Wales.
Following the sacking of Labour veteran Mandelson as ambassador last year over his ties to the late sex offender Jeffrey Epstein, Starmer had won a brief reprieve from his critics after limiting the United Kingdom's role in the Iran war.
However, on Thursday it emerged that Mandelson had failed the security vetting conducted before his appointment as envoy, a fact that Starmer's team said the prime minister had been unaware of.
Starmer's political foes have questioned how a prime minister could not know and have demanded his resignation.
Starmer, who was in France on Friday for talks on the Iran crisis, told reporters it was unforgivable that he had not been told about Mandelson having failed security vetting "when I was telling parliament that due process had been followed".
Starmer said he would "set out the relevant facts" on Monday to parliament.
A spokesman for Starmer told reporters the prime minister had no plans to resign.
Darren Jones, the chief secretary to the prime minister, said on Friday that "the recommendation was to not appoint Peter Mandelson to the role," and that the Foreign Office ignored it.
He said that was "astonishing" but within the rules.
Downing Street officials moved swiftly late on Thursday to try to quash the scandal, sacking the Foreign Office's top official Olly Robbins.
Friends of Robbins were reported by Sky News on Friday as saying the rules of the vetting procedure meant he could not pass on the concerns raised to Starmer or disclose what else had been considered when granting approval.
Yet Starmer's team's argument that he did not know until this week key information surrounding an appointment he had promoted in 2024 as a stroke of genius has sparked doubts over whether the prime minister has a proper grip on his government.
One Labour MP, speaking on condition of anonymity, said the party was unlikely to try to remove Starmer for now but that the Mandelson saga was "a gift that keeps on giving" and would ensure that he remained under scrutiny before an expected drubbing in the local elections on May 7.
Another Labour MP said David Lammy, the UK's deputy prime minister who served as foreign secretary at the time, should quit.
"The choice is incompetence over deceit," the MP added.
Starmer's spokesman said no minister and no one in the prime minister's office had known about the vetting failure.
George Foulkes, a Labour member of the House of Lords, the UK's upper chamber of parliament, urged caution, saying it would be reckless to move against Starmer.
"We need to keep things in perspective when there are so many issues he has been dealing with well," he told Reuters.
Starmer could be challenged if 20 per cent of Labour members of parliament support a rival candidate to replace him.
That means such a candidate would need the backing of 81 MPs.
The point of contention for opposition politicians is whether Starmer knowingly misled parliament when he reassured MPs that Mandelson had completed security vetting when he was appointed and that no red flags had been raised.
A letter from the Foreign Office in January last year offering Mandelson the job as ambassador, and released by parliament last month, suggested that he had passed the security vetting.
Mandelson was sacked in September when the extent of his ties with Epstein was revealed in documents published in the United States.
He is now under police investigation on suspicion of leaking government documents to Epstein but has not commented publicly on the allegations.
with AP
