The prime minister is expected to address a meeting of the Parliamentary Labour Party on Monday, amid anger over his appointment of Mandelson as ambassador to the US despite knowing that his links with Jeffrey Epstein continued after the financier's conviction for child sex offences.
It comes a day after Morgan McSweeney quit as Starmer's chief of staff, taking "full responsibility" for giving the prime minister advice that resulted in the "wrong" appointment.
But scrutiny of Starmer's own judgment is mounting as critics, including some of his MPs, have highlighted that he made the final decision.
The pressure on his premiership looks unlikely to ease as the government prepares for the lengthy process of releasing tens of thousands of emails, messages and documents relating to Mandelson's appointment.
Starmer believes the files will prove Mandelson lied about the extent of his ties to the notorious pedophile during his vetting.
The Guardian, citing a well-placed source, said they would show the Cabinet Office had warned about the grave reputational risk of handing Mandelson the ambassadorship.
Starmer and McSweeney, blamed by many for pushing for his ally Mandelson to get the coveted ambassadorship, mutually came to the decision that it was the right moment to move on, it is understood.
The prime minister credited his longtime adviser's "dedication, loyalty and leadership" for Labour's 2024 general election win and said he owed him a "debt of gratitude" in a statement that did not mention the Mandelson fiasco.
Labour MPs on the left of the party, including Brian Leishman, Ian Byrne and Kim Johnson, suggested Starmer should consider following McSweeney out the door.
Two unnamed cabinet ministers were quoted by The Times as saying Starmer was "weaker" and "could stand down at any moment", a claim No 10 said was "categorically untrue".
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