Pressure on the Labour leader had been building for a while, amid dire poll ratings and a series of damaging revelations over his decision to appoint Peter Mandelson as his US ambassador last year.
Elections to the Welsh and Scottish parliaments, as well as local councils in parts of England, in May had long been billed by gloomy Labour MPs as a last chance for him to prove he could turn things around.
In the end, the results were devastating, prompting Wes Streeting to resign as health secretary, as well as a clutch of more junior ministers.
An ally of Andy Burnham quit as MP for Makerfield, clearing a path for the then Greater Manchester mayor to fulfil his ambition of returning to Westminster to challenge for the Labour crown.
Sir Keir had previously been able to block Burnham from standing in a by-election in February. But with scores of MPs demanding his resignation in the wake of the election results, he did not have the authority to repeat the move.
His standing ebbed further when a long-simmering row behind the scenes over defence spending prompted Labour stalwart John Healey to resign as defence secretary earlier this month.
Burnham’s decisive victory in Makerfield last week, where he increased Labour’s majority over Reform UK, further strengthened his appeal to many MPs.
Sir Keir initially insisted he would contest a challenge to his leadership, despite several of his cabinet ministers privately urging him to quit.
But after mulling it over this weekend, he ultimately concluded he did not have enough support among Labour MPs to make a make such a move viable.
