Staff illness cost the Prison Service £80 million last year, according to the National Audit Office.

A report published on Wednesday found that employees spent an average 14.7 days on sick leave in 2002/03, compared to an average five years previously of 12.6 days and a target of nine days.

A total of 668,337 days were lost to sickness absence, the equivalent of a year's work for 3,000 staff.

Absences caused by stress, anxiety and depression rose dramatically, the NAO found, while the numbers caused by assaults on staff rose from 397 in 1999/00 to 693 in 2002/03.

Auditor general Sir John Bourn acknowledged that improvements in data gathering accounted for much of the rise, but said sickness levels are still too high.

"The Prison Service has made good progress in tightening up its procedures for recording sickness absence and now has a better grip in the performance of establishments," he said.

"The average sickness rate remains high, however.

"Any reduction in working days lost would free resources which could be used, for instance, to improve regimes for reducing re-offending rates or to deal with the increased number of prisoners being held in custody.

"Further reductions in the average rate of sickness absence among Prison Service staff depend on governors working more closely with staff to improve morale and to overcome a culture of absenteeism prevalent in some prisons."