LONDON — Anas Sarwar, the leader of Britain’s center-left Labour Party in Scotland, will call for Britain’s Prime Minister Keir Starmer to quit later Monday.

A senior Scottish Labour official, granted anonymity to discuss internal decisions, confirmed to POLITICO that Sarwar — once a close ally of Starmer — will call for him to resign at an unplanned press conference to be held at 2.30 p.m.

He will be the most senior Labour politician to break ranks, and will deal a body blow to the prime minister’s authority as he tries to hold on to power.

“It will be worth tuning in for,” the official added.

Starmer’s premiership hangs in the balance after fresh revelations surfaced in the Epstein files about the relationship between Peter Mandelson, the man Starmer chose for the role of ambassador to the U.S, and the convicted sex offender Jeffrey Epstein.

The prime minister has been hit by a double resignation from his top team in Downing Street. His chief of staff Morgan McSweeney — one of his most loyal, long-serving and important lieutenants widely credited with propelling Starmer first to the leadership of the Labour Party and then into Downing Street — quit Sunday, taking “full responsibility” after advising Starmer to appoint Mandelson.

He was followed out of the door Monday by Tim Allan, a former adviser to ex-PM Tony Blair, who had served as Starmer’s comms chief for six months.

As leader of the Scottish Labour Party, Sarwar is facing elections in May to lead the devolved Scottish government.

Once the clear favorite to replace the long-serving, pro-Scottish independence Scottish National Party government, Scottish Labour has slipped in the polls amid Starmer’s unpopularity.

Sarwar had tied himself closely to Starmer as he entered Downing Street, boasting about their close relationship. As Starmer became more unpopular, he distanced himself and became more critical of the PM’s administration.

This developing is being updated