The head of Britain’s largest trade union has demanded that Rachel Reeves be sacked as chancellor if the Treasury continues to hold up a multibillion-pound defence investment plan.

Sharon Graham, the general secretary of Unite, said tens of thousands of jobs were at risk from political dithering and called on ministers to “back British industry” by signing off on future defence contracts.

“If Rachel Reeves can’t grasp that concept and doesn’t care where things are made then she should go,” Graham said “Actually, you have to have a vision for Britain. You can’t just be in government, you can’t just say today’s a new day.”

She also called on Keir Starmer “to do what he said he would do” after the prime minister promised in February last year to increase annual spending on the military to 2.5% of GDP by 2027. Subsequently, Starmer promised to increase it further, to 3.5% by 2035, an extra £30bn in real terms, but few new contracts have followed.

Concerns are most acute for the Leonardo helicopter factory at Yeovil, the sole bidder on a stalled £1bn manufacturing contract. It employs 3,300 people on an average salary of £58,000 a year, and its Italian owner has said it will have to close down unless it is awarded the work before 1 March.

Adam Dance, Yeovil’s Liberal Democrat MP, said jobs across the town would be affected if the factory closed. He said a local hotel had told him it would probably have to shut, and the uncertainty was already affecting the local housing market. “Staff don’t want to commit to new purchases,” he said.

Graham was speaking to the Guardian outside Downing Street where Unite had organised a protest in response to the government’s failure to publish the defence investment plan amid Treasury resistance to its high cost.

“Labour is supposed to be in for workers in the working class. I’m seeing very little evidence of that,” Graham said. She argued this was a problem not just in terms of creating defence jobs but across the government.

The defence industrial plan had been expected to be published in the autumn, and then just before Christmas, but has been delayed to March or April. It is intended to set out funding for £67bn of commitments from last summer’s strategic defence review.

The Treasury has raised concerns about the affordability of the overall package, while the Ministry of Defence has indicated it needs an extra £28bn over the next four years to meet its forecast costs.