Starmer, who won the largest majority in modern history for Labour at a national election in 2024, faces new questions over his political judgment, just three weeks before his party is expected to be punished in local elections in England.
Following the resignation of Labour veteran Peter Mandelson as US ambassador over his ties to the late sex offender Jeffrey Epstein, Starmer had managed to win a brief reprieve from his critics after limiting Britain's role in US President Donald Trump's and Israel's war in Iran.
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 he was furious and 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".
Asked if he would resign, Starmer said he would "set out the relevant facts" on Monday to parliament.
Downing Street moved swiftly late on Thursday to try to quash the scandal, sacking the Foreign Office's top official, Olly Robbins.
Yet his team's argument that Starmer 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 move against Starmer for now but the Mandelson saga was "a gift that keeps on giving".
Another Labour MP said David Lammy, Britain's deputy prime minister who had served as foreign secretary at the time of the vetting, should quit.
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 2025 offering Mandelson the job as ambassador, and released by parliament in March, 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 under police investigation on suspicion of leaking government documents to Epstein but has not commented publicly on the allegations.
Starmer has previously apologised for appointing Mandelson, accusing the former ambassador of creating a "litany of deceit" about his ties to Epstein.
Conservative Party leader Kemi Badenoch described Starmer's defence as "preposterous" and Nigel Farage, the leader of Labour's main electoral challenger, the populist Reform UK party, said it was "blatant dishonesty".
