Workers at both of Scotland's nuclear power stations are to be consulted over industrial action in a dispute over pay and conditions.
The GMB union said it will hold a consultative ballot among its members at the Hunterston B and Torness stations in the coming weeks.
The union is in dispute with EDF Energy over pay, terms and conditions during statutory outage periods.
GMB Scottish secretary Gary Smith said: "Industrial action at Hunterston B and Torness could have profound implications for electricity generation and supply across Scotland, but we have been left with little choice than to take on EDF over these draconian cuts.
"Outages are a statutory requirement, a safety-critical process delivering the maintenance of our nuclear reactors.
"During these periods staff become fully available, work flexibly to accommodate the employer's requirements and receive enhanced rates in return.
"The outage rates were negotiated by trade unions because this is an intensive period of work that places significant demands on staff across the UK's nuclear fleet and it is ludicrous that a hugely profitable multi-national like EDF is trying to break this agreement.
"This is not a decision we are taking lightly but we hope that EDF will see sense, withdraw this cuts imposition on our members' terms and conditions, and return to negotiations."
An EDF spokesman said: "We are actively engaged in consultation with our trade union partners, including GMB, and are not aware of any ballot notifications.
"We're sharing open and constructive dialogue in a period where costs are going up and income from electricity generation is going down, and have been having productive conversations on how to achieve our shared goals."
Another ballot would have to be held before any industrial action goes ahead.
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