Workers at the Sellafield nuclear site in Cumbria are to be balloted for strike action after talks aimed at solving a pay dispute broke down.
Officials from the GMB and Prospect unions, representing 10,000 workers, met management on Wednesday to try to break the deadlock.
The unions have rejected an offer of a 2% rise on basic pay and a further 2% linked to productivity.
A spokesman for Sellafield said it was "understandably disappointed".
The GMB has now announced that a ballot for strike action will start next week with the result expected in early September.
Workers at the company's site in Capenhurst in Cheshire will also be balloted.
"The company is working within a set budget and simply cannot afford to offer any more without making efficiencies"Steve Gibbons, GMB regional officer, said Sellafield needed to improve the basic pay offer, while Prospect described the offer as the "worst in living memory".
Sellafield managers made a final offer of a guaranteed 2% pay rise with a further 2% bonus funded by efficiency savings.
A spokesman said: "We feel strongly that the offer was a fair one when taken in the context of the challenging climate in which the business is currently operating.
"The company is working within a set budget and simply cannot afford to offer any more without making efficiencies.
"The bonus element of our offer was designed to allow the workforce to share in the reward that comes with delivering more efficient working."
He said the company had reviewed a previous offer and was now offering improved benefits for the lowest paid, and a revised offer on basic pay for the entire workforce.
Safety would remain the company's number one priority at the nuclear reprocessing site during any industrial action, he added.
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