Sick leave in the National Probation Service is running at an average of 12.3 days a year for each member of staff, directly costing the service £31.6m a year, according to a damning report by the National Audit Office (NAO).
The difference between what is being achieved and the service's target of nine days amounts to 66,420 working days lost, added the NAO. This was equivalent to £11m, or about 300 full-time employees, it said.
The cost of overtime payments to cover for absent colleagues could add a further £2m a year, it calculated.
The NAO partly blamed insufficient management information, which hindered the service's ability to diagnose the reasons for high levels of sickness absence and take appropriate management action.
"The service has limited information on the causes or average lengths of sickness absence or breakdowns by age or gender," it added.
One-third of days lost were because of stress, costing £9.8m, despite the implementation of a stress management programme.
So far, just one-fifth of employees are given stress awareness training, it added.
NAO head Sir John Bourn said: "Tackling the problem will require investment in better information, robust and proactive management, and consistent application locally of policies that have been agreed nationally, particularly to tackle stress and improve work-life balance."