Labour’s biggest union backer is actively considering a formal split from the party, in a move that would deepen the crisis engulfing Sir Keir Starmer’s premiership and raise fresh questions over his ability to hold his governing coalition together.

Senior figures in Unite, Labour’s largest affiliate and historically its single biggest donor, are discussing whether to call an emergency rules conference that could bring forward a vote on disaffiliation, according to union sources. Under existing timetables, members would not be due to decide the question until the next rules conference in 2027.

People close to the talks describe “intense frustration” with the Government’s direction, running from Unite’s executive to workplace activists. Union insiders say anger has crystallised around three flashpoints: a tight, tax‑raising Budget, a heavily diluted workers’ rights package and Labour’s approach to the North Sea energy transition.

The potential rupture comes at a moment of acute political vulnerability for Starmer. Polling this autumn has recorded some of the worst approval ratings for a sitting prime minister in modern times, with large majorities telling survey firms they are dissatisfied with his performance and doubtful he can unite his party. Reform UK has pulled ahead in both voting‑intention and preferred‑prime‑minister measures, while surveys of Labour members suggest more than half would like a new leader before the next general election.

Against that backdrop, some Labour MPs believe the relationship with Unite may now be beyond repair without a change at the top. One backbencher on the party’s left said colleagues expected Unite to “coalesce around a single left candidate” if a leadership contest were triggered, arguing that the union’s support could no longer be taken for granted under Starmer.

Sharon Graham, Unite’s general secretary, has been signalling for more than a year that continued affiliation could not be assumed. She has repeatedly warned that Labour’s flagship Employment Rights Bill – sold to voters as a “new deal for working people” – has been stripped back under pressure from employers and the House of Lords. The final compromise, which delays full unfair‑dismissal protection and waters down pledges on banning zero‑hours contracts and “fire and rehire” practices, has been condemned by Unite as a pale version of Labour’s manifesto promises.

Graham has also attacked the Government’s fiscal stance. November’s Budget, which combined significant tax rises with the decision to extend the windfall levy on North Sea oil and gas profits, was described by the union as picking the wrong side between “workers and the wealthy”. Unite argues that the approach to net zero risks sacrificing well‑paid industrial jobs now without a funded, credible plan for alternative employment in green industries.

In the North Sea, the union says hundreds of posts have already gone in oil, gas and refining, and accuses ministers of failing to secure investment in new technologies at shuttered sites. Graham has likened the strategy to “letting go of one rope before you have hold of another” – a formulation that has become a rallying line for Unite workplaces directly affected by closures.

While Unite has been edging away from Labour for some time, the current discussions go significantly further than previous funding cuts and rhetorical warnings. The union has already slashed its central donations to party headquarters, diverting money towards individual MPs, mainly on the left, and to industrial campaigns. Electoral Commission records show that overall trade union contributions to Labour have fallen to some of their lowest levels in more than two decades, even as the party has courted record support from wealthy individuals and businesses.

If Unite formally disaffiliated, Labour would not suddenly be insolvent. Under Starmer, the party has built a substantial big‑donor base, with multimillion‑pound gifts from business figures helping to finance its 2024 election campaign and subsequent operations. But the optics of losing the movement that has bankrolled Labour to the tune of tens of millions over a generation would be stark, reinforcing the sense among critics that the party is drifting away from its roots towards a more technocratic, business‑friendly model.

The disquiet is not confined to Unite. Unison, another of Labour’s largest affiliates, has just elected a new general secretary after a campaign in which the union’s relationship with the Government was a central dividing line. Left‑wing challenger Andrea Egan, who argued that Labour had done “very little for working people” in office and pledged that Unison would no longer be “subservient” to the party, positioned herself as an admirer of Greater Manchester mayor Andy Burnham – a figure frequently cited by disenchanted members as an alternative national leader.

Union politics are increasingly intersecting with manoeuvring in Westminster. Recent polling of Labour members has found Ed Miliband, the former leader and current Energy Secretary, emerging as the most favoured potential successor to Starmer, well ahead of front‑bench rivals. Allies of Wes Streeting, the Health Secretary, have been reported to be quietly testing support for a future leadership bid, while Angela Rayner, removed as deputy prime minister earlier this year, remains popular among activists and many trade unionists.

For now, No 10 insists Starmer is focused on governing and delivering for “working families”, pointing to measures such as the freeze in rail fares and prescription charges, limited support with energy bills and moves to cut NHS waiting lists. Supporters argue that once MPs and members see progress on these “core Labour values”, talk of coups and splits will ebb.

Few in the movement, however, underestimate the symbolism of the moment. Since its foundation, Labour has relied on trade unions not only for money and organisers but for its basic claim to speak for organised labour. While smaller unions such as the RMT and Fire Brigades Union have walked away in the past, a break with Unite would be on an entirely different scale.

The mechanics of any divorce remain uncertain. Unite’s rulebook requires a members’ vote at a rules conference to leave Labour, and calling an emergency gathering would itself be a significant escalation. Union officials stress that no final decision has been taken. But the fact that disaffiliation is now being actively planned for – rather than merely threatened – underlines the depth of the rift.

As Starmer tries to steady a government buffeted by hostile polls and a surging populist right, the battle to keep his biggest union ally inside the tent may prove as decisive for his future as any clash across the despatch box.