Starmer, 63, had lost two of his key Cabinet ministers in recent weeks as discontent within his party grew. Health Secretary Wes Streeting quit last month and issued a stinging attack on Starmer's record of indecision, saying at the time he would consider his own leadership bid.
Another blow was dealt this month when Defense Secretary John Healey quit in a dispute over military funding, accusing Starmer of failing to deliver on his public promises on the issue.
Among Starmer's lingering political headaches has been a damaging scandal over his decision to appoint Labour grandee Peter Mandelson as U.S. ambassador in December 2024 despite Mandelson's friendship with the late convicted sex offender and financier Jeffrey Epstein.
Starmer eventually sacked him after Mandelson's personal messages with Epstein came to light, but the prime minister was forced to deny lying to lawmakers following a series of questions about the vetting process.
Mandelson was arrested in February on suspicion of misconduct in public office connected to Epstein, though so far no charges have been brought. He has previously denied all wrongdoing.
The scandal proved especially difficult for Starmer given his 2024 election promise to clean up politics, one of several pledges that have since come back to haunt him.
After years without sustained growth in Britain, economic headwinds linked to global conflicts and the fallout from Brexit as well as his own electoral pledge of fiscal discipline have left a leader who promised change with little room for major reforms or investment in ailing public services. Instead, Starmer has been left grappling for ways to cut spending and hike taxes while complying with his ironclad promises not to do so.
Meanwhile, a series of Starmer's flagship political projects, from massive investment in net-zero emissions to the rollout of digital ID cards, have ended up dramatically watered down or scrapped entirely — fueling doubts even among some of his allies about his propensity for tactical mistakes and miscalculations.
“In one sense, it was always going to be tough for Starmer,†said Andrew Barclay, a politics lecturer at the University of Sheffield.
“The economic legacy that the Labour government inherited was obviously very tricky to begin with,†he told NBC News, “but it needn't have been as bad as it has been.â€
Starmer has “never really properly been able to establish, at least in the minds of the public, what it is the government is actually trying to do in the first place,†he added. “Scandals are always important, but they're all the more important when a government doesn't have a central narrative to go back to.â€
Tim Bale, a politics professor at Queen Mary University of London, said the seeds of Starmer's downfall were sown before he took power.
Starmer made a “whole bunch of promises†to progressives to win control of his party but then steered to the center, leaving his government “without the financial means†to bring about lasting change, he told NBC News.
He also rolled out a series of controversial policies that alienated progressive voters while chasing right-wing approval on immigration, he said, adding: “It was all downhill from there.â€
While struggling at home, Starmer has earned praise for his response to global conflicts, sometimes appearing more at home at international summits than when wrestling with the details of domestic policy. His initially warm working relationship with Trump was soured by his decision to keep out of the Iran war, a stance that even Starmer's detractors have since called brave and principled.
In the now looming leadership contest, candidates need the support of 20% of Labour MPs to be considered. If more than one clears that threshold, a vote will be held among party members and supporters.
But Burnham is the overwhelming favorite, and some within the party have expressed hopes of avoiding a messy, drawn-out contest if he is the only serious challenger.
Starmer originally promised to fight in any leadership contest, but the scale of Burnham's victory in Makerfield against Farage's Reform party was “the straw that broke the camel's back,†added Barclay.
“Andy Burnham's stock is so high at the moment, and it's just implausible, really, that Starmer could feasibly fight on,†he said. “This is a way of the prime minister exiting with as much grace as he possibly can.â€
And by winning the Makerfield by-election so emphatically, Bale said, Burnham proved he has what it takes “to take on Nigel Farage, and that he has the communication skills and charisma that Starmer so obviously lacks.â€
The next prime minister will be the country's seventh in just 10 years, a prolonged period of instability that began with the 2016 Brexit vote.
The son of a toolmaker and a nurse and the first in his family to go to university, Starmer regularly describes his background as “working class.â€
He would go on to become a high-profile barrister, rising to become director of public prosecutions, the most senior criminal prosecutor in England and Wales.
He became an MP in 2015 and was elected Labour leader after the party's heavy defeat in the 2019 general election, reviving the party's fortunes and leading it to a commanding victory just four years later.
