Almost £110 million has been spent hiring locum doctors to provide temporary cover in Northern Ireland hospitals, auditors have found.
Around two-thirds of the bill - £74 million - was paid to external recruitment agencies with the remainder paid to current NHS doctors for them to work extra shifts and overtime, the Northern Ireland Audit Office report discovered.
The £109 million cost of locums is 8% of the £1,360 million spent on medical staff.
But in certain areas the spend accounts for a much higher percentage of the salary bill - 11% in the Northern Trust and 17% in the Western.
Auditors said this was down to challenging local circumstances, including difficulties in filling vacancies.
Comptroller and Auditor General Kieran Donnelly said if all trusts could keep within the 8% regional average then the health service would save £5 million a year.
The report also found: a wide variation in the hourly rates agencies charge for locums; internal locums often seek to barter for rates above those approved by the health department; inconsistency in the way trusts screen and induct locum doctors and the way they manage their employment.
Mr Donnelly said trusts have acknowledged that regional collaboration in relation to the appointment of locums could create greater efficiencies and a plan has been developed to establish a regionally managed service.
The auditor general said there were certain problems associated with employing locums.
"Although the use of locum doctors allows hospitals to maintain appropriate and safe staffing levels, it also creates potential risks," he said. "Locum staff may be unfamiliar with a hospital or its procedures and may not be in a position to offer continuity of care."