Union leaders representing thousands of nuclear workers are to discuss calling a strike ballot in a row over pensions.
The unions said 16,000 workers at 19 sites face cuts under plans by the Nuclear Decommissioning Authority to make savings of £660 million.
The workers are based at Sellafield (Cumbria), Magnox (Anglesey), Ayrshire, Dorset, Dumfriesshire, Essex, Gloucestershire, Gwynedd, Kent, Oxfordshire, Somerset, Suffolk, Direct Rail Services (Cumbria), Dounreay Site Restoration Ltd (Caithness), Low Level Waste Repository (Cumbria) and International Nuclear Services (Cumbria, Warrington).
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The unions said the Government's expectation is that the final salary pension schemes in place across the NDA estate will be reformed by April 2018.
Justin Bowden, GMB national officer, said: "There is no justification for this attack on the pensions of these nuclear workers and their communities.
"These pension funds are in a sound state and underwent considerable reform 10 years ago.
"What the Government are saying is that the privatised site license companies who run these nuclear facilities are in fact public sector organisations and therefore Sellafield, Dounreay and the Magnox sites should go through the same reforms as the rest of the public sector itself."
Kevin Coyne, Unite national officer, said: "We are urging all our members working for the Nuclear Decommissioning Authority (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.
"It is blatantly clear that the NDA is the stalking horse for the Government is which hell-bent in saving £660 million over the period of decommissioning."
Dai Hudd, Prospect's deputy general secretary, said: "Prospect members are some of the most highly skilled workers in the UK, indeed some number amongst the highest skilled engineers and scientists working in this field in the world.
"There can be no justification for these cuts to the future pensions of these workers."
The unions will meet next Monday.