Around 1,400 contract workers at the Sellafield nuclear site in Cumbria have staged an immediate walk out.
A Unite union spokesman told ITV Border the unofficial action at Sellafield was taken first thing this morning following a meeting with the contractors at 7am.
The workers will be off site until Monday, and the strike action will cost them each around £500.
Changing rooms are at the centre of the row, with Unite claiming they are not up to standard, and that dirty towels are left for them to use after showering.
In a statement, Sellafield say they have taken measures to ensure safety at the nuclear complex.