“Serious industrial unrest” at Europe’s biggest nuclear site could threaten the Conservatives’ chances of winning a forthcoming byelection, unions have warned.

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.

“If the NDA proceeds with its proposed consultation in its current form there will inevitably be a significant reaction from the members affected. The likelihood of serious industrial unrest cannot be ruled out.”

The two money-saving proposals on offer involve either a series of changes including increasing the pension age from 60 to 65 or state pension age (whichever is higher), or breaking the final-salary link for the pension scheme, according to Prospect. A 60-day consultation period on the options opens on 9 January.

According to Hudd, either proposal will affect thousands of Sellafield employees as well as thousands of employees at other nuclear sites, some of which are also in the constituency. Each member of the scheme would lose tens of thousands on average, he claimed.

“I expect the reaction will be particularly robust because this group of members were granted statutory pension protection in the legislation that effectively privatised the industry and these proposals would mean overriding those protections,” he told the Guardian.

“There are few constituencies where a single industry (indeed employer) is as significant as the nuclear industry and Sellafield is to Copeland. For the government of the day to attack the pension terms for the employees in this industry in the run-up to a crucial byelection, there is incredibly bad timing to say the least.”

A spokesman for the NDA said: “Government policy is that all public sector final-salary pensions schemes should reformed by 2018, and 4 million public sector workers have already moved to new pension arrangements.



“Specific decisions on how to change the Nuclear Decommissioning Authority’s estate pension schemes have yet to be taken. We expect to begin formal consultation in the new year.”

More than 10,000 people are employed at the Sellafield site, which measures 6 sq km and is the largest nuclear site in Europe, containing more than 1,000 nuclear facilities.

Almost half of the UK’s nuclear workforce is based at Sellafield, which is home to among the largest inventories of untreated waste in the world.

The NDA purpose is to deliver the decommissioning and cleanup of the UK’s civil nuclear legacy in a safe and cost-effective manner.