Workers at the Sellafield nuclear reprocessing plant in Cumbria have voted in favour of strike action in a row over pay.
The site's industrial workers want their rates of shift pay brought into line with the company's white-collar employees.
They are looking to increase their pay by about £2,000 a year.
The GMB union says the owners of the plant, British Nuclear Fuels Limited (BNFL) had agreed in 1999 to end shift pay discrepancies by 2004.
The union is angry this could now be delayed until 2009.
Brian Strutton, GMB National Secretary for Education said: "The workers are feeling let down that their employer has reneged on the agreement and aren't willing to wait any longer.
"We have offered further negotiations but this has met with a refusal from BNFL."
Members of the GMB and Amicus have overwhelmingly backed industrial action in a ballot of 2,400 workers. Unions will meet on Tuesday to discuss whether to set strike dates.
A BNFL spokesman said: "We are very disappointed that, despite discussions with national officers, the Sellafield shop stewards wish to reject the company's final offer and enter into a ballot for strike action."
The spokesman said safety at Sellafield would be a top priority during any industrial action.