Sir Keir sacked Lord Mandelson in September, after emails emerged showing he sent supportive messages to Epstein as he faced charges for sex offences.
But further revelations this week from the latest release of Epstein files in the US about his continued contact with the late disgraced financier after his conviction in 2008 have led to fresh fury within Labour over the decision to hand him the role.
Emails suggesting the peer leaked sensitive government information to Epstein whilst a minister in the last Labour government have also prompted a police investigation into alleged misconduct in public office.
Sir Keir has said Lord Mandelson’s prior relationship was Epstein was flagged when he was vetted for the role, whilst insisting the former cabinet minister misled him about the depth of the relationship.
But it has failed to contain the row over the decision, with a handful of Labour MPs now openly calling for Sir Keir to resign as party leader.
The prime minister is expected to publicly address the episode as soon as Monday, as he seeks to convince his backbenchers he can regain control of the situation.
And he will address Labour MPs in private on Monday evening, at the weekly Parliamentary Labour Party meeting.
It comes as the boss of the Fire Brigades Union, Steve Wright, became the first leader of one of Labour’s 11 affiliated unions to call for the prime minister to go, adding “everybody’s thinking it”.
McSweeney’s departure is a big blow to Sir Keir, with the 48-year-old Irishman having masterminded his successful bid for the Labour leadership in 2020 and the party’s campaign for the general election in 2024.
His deputies Jill Cuthbertson and Vidhya Alakeson have been appointed acting chiefs of staff in the wake of his exit, Downing Street has confirmed.
