Firefighters at a huge nuclear decommissioning site began a strike today over pay and working conditions.
Members of the GMB union said they walked out of Sellafield in response to its management's "broken promises".
Workers are striking between 6am and 6.30pm today and at the same time this Thursday.
Strike action was previously averted in July after last ditch talks, and further action was recently suspended to allow management time to implement a resolution, which GMB said was to be in place by the end of September.
"Management have broken their promises to the firefighters – and have failed to make a fair offer that would put an end to this dispute," said Chris Jukes, GMB senior organiser.
GMB called the promised resolution "stalling tactics".
"Despite the apparent progress and promises more two months ago, it appears senior management in Sellafield Limited have gone bent on their undertakings and returned to the incompetent communications and negotiating style that has become the way of some top brass at Sellafield in recent years.
“GMB members feel insulted that management have taken this approach and even more incensed when management have expressed ‘disappointment’ with the response from GMB negotiators to the latest offer."
Sellafield warned staff GMB firefighters were intending to picket the gates and said they had done all they could to minimise disruption and most work could continue as normal.