About 16,000 workers at 19 UK nuclear sites are to be balloted on strike action in a dispute over pensions.
The workers belong to four different trade unions and are mainly employed in decommissioning work.
They are angry at plans by the Nuclear Decommissioning Agency (NDA) to reform their final-salary pension schemes.
They say the plans by the NDA, aimed at saving £660m, will reduce their retirement incomes by thousands of pounds.
Sites affected include Sellafield in Cumbria and Dounreay in Caithness.
Other workers involved work for Magnox at sites in Anglesey, Ayrshire, Dorset, Dumfriesshire, Essex, Gloucestershire, Gwynedd, Kent, Oxfordshire, Somerset and Suffolk.
Kevin Coyne of the Unite union, which is one of the four involved, said: “We are urging all our members working for the NDA to resist this proposed Treasury-led raid on their pensions.
“If it is allowed to go ahead, thousands of workers will see their retirement incomes slashed by thousands of pounds.”
The GMB, Prospect and Aslef are also balloting their members.