Workers at the company that provides the warheads for Trident, the country's nuclear deterrent, will stage a protest tomorrow in a row over pay.
Prospect said its members at Atomic Weapons Establishment (AWE) sites in Aldermaston and Burghfield in Berkshire will hold a mass meeting before taking action short of a strike.
The union said a current pay offer of less than 2% for most of its 1,300 members did not reflect the success of the organisation.
Negotiations officer Bob King said: "Members have had poor pay settlements imposed on them, seen an ongoing increase to their pension costs and faced redundancies, yet still maintain the very high standards expected at the organisation.
"We cannot emphasise enough the need to resolve this issue as soon as possible. Our members do not want to be engaged in industrial action, they want to do their jobs, but they need to do so in the knowledge that their employer will treat them fairly.
"We remain willing to meet with AWE at any time to reach a conclusion that will benefit all parties."