The Garda Ombudsman Commission has launched a strong attack on the force for its lack of co-operation with an investigation into links between gardai and a convicted drug dealer.

All officers concerned have been cleared of any criminal wrongdoing - but the Commission says the delay in releasing evidence seriously impacted its investigation.

The report also criticises gardai for failing to fully implement informant-handling protocols.

The Commission says its 4-year investigation has hampered from the outset by poor cooperation. It says it received evidence in a piecemeal fashion, with only 17 of 36 requests being serviced within the 30-day deadline.

The report highlights grave concerns over Garda handling of informants - both historic and current. It also says there was culture of non-adherents to protocols, including the keeping of proper records of meetings with informants.

On the basis of this report, the Director of Public Prosecutions (DPP) has decided against pursuing criminal proceedings against any gardai.

The Garda Commissioner and Justice Minister have received the results of the investigation with a list of concerns and recommendations given to Minister Alan Shatter.

He must publish these as soon as reasonably practical.

Garda Ombudsman Commissioner Kieran Fitzgerald says all this had an impact on the investigation.