Up to £23 million was wasted paying lawyers for complex legal cases which failed to last as long as expected, the Northern Ireland Audit Office has said.
The money was paid since 2005 for criminal matters which did not go to a trial taking more than 25 days, auditor general Kieran Donnelly added. Only a tenth of cases lasted as long as expected.
The practice for very high-cost cases (VHCCs) has been abandoned but it could take up to three years before payments still in the system are made.
Some solicitors have refused to take cases to crown courts in a dispute over changes to fees which has caused disruption to the legal system.
Mr Donnelly said: "With such sums of money involved, it is important that the current framework for managing criminal legal aid ensures value for money for the taxpayer and proper accountability. At present, that is not the case."
Since 2003, the Northern Ireland Legal Services Commission has spent £340 million on criminal legal aid which is paid to cover the cost of defendants' counsel.
In a major change, the Courts Service has scrapped VHCC cases and introduced a standard rate. Previously, cases which were expected to last more than 25 days attracted higher rates of remuneration for preparatory work and higher fees. The commission had no power to remove the high cost classification or reduce the fees payable if it did not last longer than 25 days.
However, cases can be abandoned for a variety of reasons, including good legal work which creates enough doubt of guilt to end proceedings or which prompts a plea bargain. This preparatory work is often no less complex than cases which last more than 25 days.
The Audit Office said, comparing the amounts paid at higher rates and those payable under the standard rates, up to £23 million may have been paid since 2005 for cases which failed to go to trial lasting more than 25 days.
The auditor general said: "Considerable scope remains to improve efficiency and achieve cost savings, but progress will depend on the Department of Justice, Court Service, the commission and professional bodies working together to achieve this."