The byelection in the marginal Cumbrian seat of Copeland has been described as “Theresa May’s to lose”.

But the Conservative candidate hoping to overturn Labour’s 2,564 majority will have to explain to thousands of workers at the Sellafield nuclear reprocessing site why the government is trying to downgrade their final-salary pension scheme.

Trade unions representing many of Sellafield’s 10,000 workers have written to the government warning they cannot support either of the options being considered.

The Guardian has seen a letter sent shortly before Christmas to Lady Neville-Rolfe, minister of state at the business department. It comes from the Prospect union, which represents more than 5,000 Sellafield engineers and specialists.

The letter, signed by Prospect’s deputy general secretary, Dai Hudd, on behalf of his union, the GMB, Unite and Aslef, tells the minister “serious industrial unrest” cannot be ruled out by workers employed by the Nuclear Decommissioning Authority.

The NDA is the public body that owns Sellafield, a huge site in Copeland that processes nuclear waste from the old Windscale nuclear power station, where a fire in 1957 caused the UK’s worst nuclear accident.

It says: “Employees across the NDA estate fought hard to secure the statutory pension protections that currently apply. There will be an understandable adverse reaction with any proposals that trample over those protections.

“They will certainly not respond well to a raid on their pension benefits intended to achieve arbitrary savings agreed between the NDA and the Treasury, and agreement to which the workforce and their representatives played no part.