The Comptroller & Auditor General says some prison officers are abusing sick leave privileges.
The Comptroller's report shows that the average number of sick days taken by prison officers rose from 19 days in 2002 to 26.36 days per head in 2006.
But the figures actually include rest days between sick leave days taken, and, therefore, do not reflect the actual amount of working days lost.
The Comptroller & Auditor General says the link between sick leave and overtime needs to be severed.
It believes the introduction of annualised hours in 2005 has helped to start reverse the trend.
The system means that staff are now contracted for a defined number of overtime hours a year that cannot be exceeded.
The latest figures show that levels of sick leave was down over 10% for the first four months of 2007. The average amount of sick leave days per head for 2007 is expected to fall to 22.92 days.
