Andy Burnham, Mayor of Greater Manchester, is reportedly preparing to challenge Keir Starmer’s leadership of the Labour Party following a potential victory in the Makerfield by-election. Sources close to Burnham’s campaign indicated that the resignation of John Healey as defence secretary last week marked a pivotal shift in strategy, increasing the likelihood of an immediate leadership bid.

Healey’s departure, along with the resignations of armed forces minister Al Cairns and two parliamentary aides, was triggered by dissatisfaction with Starmer’s defence investment plan. Healey publicly criticised the plan as insufficient for the country’s defence needs during a period of heightened global threats, accusing the prime minister of failing to “meet the moment.” His unexpected resignation was described within Downing Street as a significant blow, given his prior reputation as a loyal cabinet member.

Initially, Burnham’s team was divided on the timing of a leadership challenge. Louise Haigh, managing Burnham’s Makerfield campaign, and Lucy Powell, Labour’s deputy leader, had preferred a longer timeline, planning a challenge later in the year. However, the succession of resignations shifted this view, prompting Burnham’s allies to consider launching their bid as soon as the by-election results were announced. One source indicated the campaign might declare an immediate leadership challenge on election night, while others preferred focusing on the by-election victory itself.

After the poll outcome, Burnham is expected to seek the nominations of at least 81 Labour MPs to formalise a leadership bid. Supporters argue that moving quickly would capitalise on Starmer’s recent vulnerabilities, citing the prime minister’s weakened position following the defence plan controversy.

The defence investment plan itself has been a source of intense debate. Starmer had aimed to unveil an ambitious upgrade to the UK's defence budget, but Healey’s resignation forced the government to reconsider the proposal. Dan Jarvis, who was appointed defence secretary following Healey’s resignation, has been granted two weeks to revise the plan with a potentially increased funding allocation. Critics contend the initial plan’s increase, projected at 0.08 percentage points of GDP, was inadequate, amounting to roughly a £10 billion real-terms boost rather than the stated £13.5 billion.

The timing of a recent military operation—the seizure of a Russian shadow fleet tanker, Smyrtos, by Royal Marines—prompted speculation about possible links to the defence plan fallout. Government officials denied any connection, emphasizing that the operation was planned over several weeks and approved through complex cross-departmental coordination well before the defence internal dispute.

Culture Secretary Lisa Nandy affirmed ongoing government discussions to enhance defence spending and rejected Healey’s assertion that Starmer’s administration was neglecting military funding. Nandy highlighted resource shifts from other departments to meet defence priorities and expressed confidence in Jarvis’s ability to deliver on the government’s commitments.

The renewed defence funding debate has also reopened discussions about potential welfare spending cuts to finance increased military expenditure. Burnham has publicly stated he would not hesitate to reduce welfare spending to bolster defence budgets if elected prime minister, citing changing global security conditions as justification.

Starmer’s original plan had been to use national security as a key theme during the summer ahead of international forums such as the G7 summit in France and the NATO summit in Turkey. However, Labour insiders suggest the defence plan controversy has eroded Starmer’s credibility and political standing, intensifying calls within the party for a leadership challenge.

Meanwhile, Labour activists and MPs continue door-to-door campaigning in Makerfield, and Burnham’s team is reportedly preparing detailed policy plans outlining his first 100 days in office should he succeed in unseating Starmer.