Starmer, who won the largest majority in modern history for Labour at the 2024 general election, faces fresh questions both over his judgment and his ability to govern, 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 accused him of misleading parliament and have demanded his resignation.
Senior minister Darren Jones said Starmer was furious over not having been told about Mandelson's failure to pass the security vetting.
"I don't think it brings the prime minister's future into question," Jones told LBC radio on Friday, acknowledging the system that had meant Foreign Office officials failed to communicate the vetting failure to ministers had "undermined the prime minister and the government".
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 grip.
One Labour MP, speaking on condition of anonymity, said while it was unlikely the party would move against him now, the Mandelson saga was "a gift that keeps on giving".
Another Labour MP said David Lammy, Britain's deputy prime minister who 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 Mandelson had passed the security vetting.
Mandelson was sacked in September when the extent of his ties with Epstein were revealed in documents published in the United States.
He is under police investigation on suspicion of leaking government documents to Epstein.
Conservative Party leader Kemi Badenoch described Starmer's defence as "preposterous", telling BBC Radio "the prime minister is taking us for fools".
Starmer has previously apologised for appointing Mandelson, accusing the former ambassador of creating a "litany of deceit" about his Epstein ties and promising to release documents on how he was appointed.
