There has been widespread exploitation of the fixed charge penalty notice system by senior gardaí, according to a report by the Garda Síochána Ombudsman Commission.

The report contains the findings of an investigation by GSOC arising out of complaints by Sergeant Maurice McCabe about abuse of the system.

It found 442 members of the force were authorised to cancel the notices in four years.

The report says senior officers breached policy by carrying out widespread cancellations outside their own areas.

One senior member of the force cancelled 744 notices across 17 counties.

It also says the credentials of retired senior gardaí, who had been authorised to cancel fixed charge notices, were used to authorise cancellations.

The report adds that 72% of all cancelled fixed charge notices were simply recorded as "cancelled", with no further explanation given.

GSOC began its investigation into the abuse of the penalty points system almost four years ago and examined the five years from 2009 to 2014.

It met five times with Sgt McCabe and says its findings confirm his information in relation to improper cancellations.

However the report looked at the system rather than individual cases.

GSOC said it has now closed its investigation and will not pursue criminal or disciplinary proceedings because the cost would outweigh the benefit.

The lowest quote it received from an outside agency was over a €1m with a real risk it could cost more.

The force's cancellation policy has since been changed and authorisation to cancel penalty points is now restricted to a small number of officers.

The Minister for Justice Charlie Flanagan said he accepts the conclusions drawn in the report that continuing the investigation further would not be in the public interest in light of the continuing progress and oversight of the fixed charge penalty system.

"A more robust system of supervision and continuous improvement would better serve to instill public confidence, as the report indicates," he added.

In a statement, An Garda Síochána welcomed GSOC's finding that the "procedures for dealing with fixed charge notice cancellations have dramatically changed in recent years".

The statement said that gardaí will continue to closely monitor compliance with the revised policies and procedures relating to the fixed charge processing system to ensure the high levels of compliance are maintained and there is public confidence in the system.