The fate of a prime minister can turn on the result of a single by-election. When Andy Burnham crushed Reform UK in the Makerfield by-election on Thursday night, the pressure on Sir Keir Starmer to set a timetable for his departure became overwhelming, with cabinet ministers and Labour grandees now openly calling for him to go.
At its simplest, a Labour leadership challenge is a process by which the party can replace its leader—and therefore, if the party is in government, the prime minister. Under Labour's rules, a challenger must secure nominations from 20% of Labour MPs (currently around 100) to trigger a full contest. Once that threshold is met, the party holds a ballot of all members, with the winner becoming leader. If the sitting leader resigns before a challenge, the contest can become a coronation for a single candidate if no one else comes forward.
“Explains how Labour leadership challenges work, triggered by Andy Burnham's by-election win.”
The current crisis follows a pattern that has become familiar in British politics: a leader who won a thumping general election majority—Starmer secured a majority of 170 in 2024—finds himself fatally weakened by a combination of poor local election results and a high-profile by-election defeat for his own party? Actually Burnham won for Labour, but that victory has turned against Starmer because Burnham is seen as the natural successor. The by-election result made Burnham's leadership ambitions credible, and within hours multiple cabinet ministers—including Transport Secretary Heidi Alexander, Foreign Secretary Yvette Cooper, and Energy Secretary Ed Miliband—had urged Starmer to set out a timetable to leave. Former deputy leader Harriet Harman said “the herd is not just moving against Keir Starmer, it’s stampeding”. Former home secretary Alan Johnson told LBC: “It’s over, Keir; Andy is going to stand, and he’s going to win.”
For UK readers, the immediate practical impact is uncertainty at the top of government. A leadership contest can take several weeks, during which policy decisions stall and the government's agenda risks being overshadowed by internal party drama. The pound and financial markets often react to such instability. Moreover, the prospect of a new prime minister—Burnham is expected to travel to London on Monday to meet MPs—means potentially major shifts in policy direction, from levelling up to the NHS and immigration. The last Labour leadership change, when Starmer succeeded Jeremy Corbyn in 2020, took about three months.
Q: How many Labour MPs have called for Starmer to go? Sources say around 100 Labour MPs have already told the prime minister he needs to resign. About 200 MPs are reportedly prepared to sign Burnham's nomination papers if a challenge is required.
Q: How quickly could Andy Burnham become prime minister? If Starmer resigns and Burnham is the only candidate, a coronation could happen within weeks. If there is a contested leadership election, the process typically takes two to three months, but MPs hope to avoid a lengthy contest.
Q: What happens if Starmer refuses to resign? Cabinet ministers have given him until the end of the weekend to set a timetable. If he does not, a meeting of senior ministers on Tuesday could see open calls for him to go. It is understood that if Starmer resists, MPs could force a no-confidence vote in the leader, which would trigger a formal leadership contest.
What happens next is finely balanced. Starmer has said he will “fight any challenge” and will not walk away. But with his own cabinet now openly briefing against him, the momentum behind Burnham appears unstoppable. The next 48 hours are critical: if Starmer does not announce a timetable by Monday, Burnham’s allies are expected to begin the formal process of a leadership challenge. Tuesday’s cabinet meeting could be the moment when the last vestiges of support for Starmer collapse.