The bill for settling medical negligence cases in Northern Ireland over the last five years is set to top £250 million, auditors have found.
The Department of Health has already paid out £116 million in damages (£77 million) and legal costs (£39 million) for claims made since 2007, and it estimates it will spend a further £136 million to cover those that have not yet been settled.
The Northern Ireland Audit Office (NIAO), which collated the figures, said the actual financial impact on the Health Service was much harder to estimate as the cost of additional patient treatment, to remedy mistakes that had been made, also had to be considered.
Auditors examined the issue as part of an overall assessment of service safety provided by Northern Ireland's Health and Social Care (HSC) Trusts.
They found that around 83,000 so-called adverse incidents of potential harm to the patient are recorded across the trusts each year - with many more going unreported.
The auditors said trusts had made progress creating a workplace environment where staff did not fear reporting such incidents, but said more had to be done to encourage openness.
Auditors found that in some cases legal costs exceeded the amount of damages ultimately paid out.
Noting the substantial sums involved, the NIAO report recommended trusts work to develop a formal resolution process that would offer patients a viable alternative to pursuing legal action.
It also advised that a region-wide system of collecting data in regard to adverse incidents and near misses was developed.
The report was launched by Comptroller and Auditor General Kieran Donnelly.
"Overall, we enjoy high standards of care from Northern Ireland HSC Trusts," he said.
"However, reducing adverse incidents that cause, or could have caused, unexpected harm to patients and clients is a core task for the Department of Health, Social Services and Public Safety and HSC Trusts.
"Two factors are crucial to this: the establishment of a culture in which incidents can be reported easily, honestly and without fear of blame; and the ability to ensure that lessons learned from these incidents are successfully taken on board by HSC staff.
"Today's report shows that while the Department and HSC Trusts have made progress in both these areas, there is more to be done.
"Whilst reporting of adverse incidents has improved at the local level, the Department accepts that under-reporting of incidents continues.
"At the regional level a reporting and learning system exists for serious adverse incidents but a regional system to ensure the effective evaluation of numbers, types and causes of all adverse incidents has still to be introduced."