Prisons in Northern Ireland are facing disruption after the Prison Officer's Association said it was indefinitely withdrawing all overtime in a dispute over pay.
The escalation of the dispute follows a number of two day overtime bans in recent months.
The Prison Service said the action was "likely to cause disruption to the level of regime that can be provided" in each of its prisons.
POA chairman Finlay Spratt said two years ago they made a deal with the Prison Service management to inflation-proof their annual pay award.
"We were happy with an award to match inflation, you can't expect any more these days, but they have reneged on the deal.
"We are not looking for an inflation busting pay rise, we are looking just looking for the cost of living rise they promised.
"They promised us that our award would be inflation-proofed and consolidated. Last year we got 1.63%, that was below inflation and it was not consolidated."
At the same time prison officers in England and Wales got 2.5% last year, he said.
"They have offered 0.5% for next year and that does does not meet inflation. "They shafted us, they misled us. We don't want to take action that will affect the prisoners but we have no alternative, we are ready to negotiate but they are not listening," said Mr Spratt.
The effects on the prisons regime is likely to be swift. Mr Spratt said they were 116 officers below strength and each officer was doing an average of 10-12 hours overtime a week.
The Prison Service said there were talks between the two sides at the Labour Relations Agency last week and both sides had agreed to meet again this week.