Thousands of workers at the Sellafield nuclear reprocessing plant are voting on whether to take industrial action.
The Unite and GMB unions, which together represent half of the waste facility's 10,000 staff, have described a pay offer of 1.5% as "completely unacceptable".
Sellafield Ltd said the award was "fair, reasonable, and affordable", and had been accepted by another union.
The ballot has now opened and the result will be announced in September.
Chris Jukes of the GMB said: "Pay at Sellafield for all but the top brass has failed to keep pace with inflation in recent years ... at the same time, Sellafield management have chipped away relentlessly at terms and conditions.
"In the circumstances, we now have no alternative but to ask our members if they are willing to take strike action in order to achieve a pay offer in line with inflation."
A Sellafield spokesman said: "We've offered our workforce an unconditional 1.5% increase this year - which is an increase to our wage bill of over £12m year-on-year.
"Like all publicly funded organisations we have to control costs and insure we are delivering value for money for the tax payer, so we're implementing a pay award which we feel is fair, reasonable and, most importantly, affordable."