The PSNI have been called in to run prisons in the Six Counties after warders failed to report for duty.
All visits to Maghaberry jail in County Antrim, where most republican and loyalist prisoners are held, were immediately cancelled.
Prison Service director Peter Russell confirmed three quarters of prison staff, some 350, had simply not reported for duty after lunch today.
He said: ``This action is reprehensible. It increases the potential for disruption in the prisons when there is already a volatile atmosphere.''
It is believed the unofficial action, which also affected Magilligan prison in County Derry, is linked to a row between management and warders over ``stress levels'' and the cost of increased security to deal with loyalist threats.
Even though some officers have remained in work, Mr Russell said police have also been brought into the regimes.
``A significant number of prisons staff have failed to complete their rostered duties this afternoon and the police service is now contributing to the day to day running of the prisons,'' he added.