Prison officers are planning further work stoppages at jails across the country in the continuing row over clocking-in arrangements.
Prison officers claim new security screening measures will lengthen their working day.
Prison warders staged their second one-hour work stoppage in jails today, with two further protests planned for next week.
The dispute centres on the introduction of new security measures to crack down on contraband being smuggled in to inmates.
The Prison Officers' Association (POA) says it supports increased security but insists a new system of clocking-in after staff are searched will lengthen their working day.
POA general secretary John Clinton said officers regretted the disruption. "Nobody wants to be in a position where you have to take a work stoppage, but unfortunately this dispute was generated by the management when they unilaterally decided to move these clocks from the entrance of the prison to an area behind the staff searching, which in effect means we have to be searched on our own time and lengthen our working day, and that's just not acceptable," he said.
Management proposed that staff be allowed an extra five extra minutes for security checks, but Mr Clinton said that was not enough time.
Officers nationwide staged a walk-out just over a week ago. Further stoppages took place today between 2pm and 3pm, with more planned for next week.
The POA said it is open to third-party intervention to resolve the row. Further stoppages are scheduled for next Wednesday and Friday.