The boss of Unite, Labour’s biggest union funder, has threatened to break its link with the party unless it changes direction.

Sharon Graham, general secretary of the union, told Sky News that, on the eve of a crucial party conference for the prime minister, Unite‘s support for Labour was hanging in the balance.

She told Sunday Morning With Trevor Phillips: “My members, whether it’s public sector workers all the way through to defence, are asking, ‘What is happening here?’

“Now when that question cannot be answered, when we’re effectively saying, ‘Look, actually we cannot answer why we’re still affiliated’, then absolutely I think our members will choose to disaffiliate and that time is getting close.”

Asked when that decision might be made, she cited the budget, on 26 November, as “an absolutely critical point of us knowing whether direction is going to change”.

Ms Graham, who became leader in 2021, has been a long-time critic of Sir Keir Starmer‘s agenda, accusing him of lacking vision.

The union has campaigned against his decision to cut winter fuel allowance for pensioners – which was later reversed – and has called for more taxes on the wealthy.

But the firm threat to disaffiliate, and a timetable, highlights the acute trouble Sir Keir faces on multiple fronts, after a rocky few months which have seen his popularity plummet in the polls and his administration hit by resignations and scandals.

There is now open discussion about his leadership, with Andy Burnham, the mayor of Greater Manchester, claiming he’s been urged by MPs to mount a challenge.

Unite has more than a million members, the second-largest union affiliated to Labour. It donates £1.5m a year from its membership fees to the party.

The union did not make an additional donation to Labour at the last election – as it has done previously – but was the biggest donor to its individual MPs and candidates. It has donated millions to the party in the past.

Any decision to disaffiliate would need to be made at a Unite rules conference; of which the next is scheduled for 2027, but there is the option to convene emergency conferences earlier.

Just 15 months into Sir Keir’s premiership, in which he has promised to champion workers’ rights, Ms Graham’s comments are likely to anger the Labour leadership.

Read more: Now is moment of ‘maximum danger’ for Starmer, Harriet Harman warns

Unite, earlier this year, voted to suspend former deputy prime minister Angela Rayner of her union membership because of the government’s handling of a long-running bin strike in Birmingham.

Ms Graham has described the left-wing party being launched by Jeremy Corbyn as a “sideshow” and has brushed off speculation of a leadership challenge by Mr Burnham.

This summer, she said if Unite dropped support from Labour it would “focus on building a strong, independent workers’ union that was the true, authentic voice for workers”.

The annual Labour Party conference kicks off in Liverpool from Sunday.

As a union affiliated with Labour, Unite has seats on the party’s ruling national executive committee and can send delegates to its annual conference.

Watch the full interview with Sharon Graham on Sunday Morning With Trevor Phillips from 8.30am on Sky News