The Chief Inspector of Criminal Justice in Northern Ireland has called on the PSNI and Police Ombudsman's office to put in the hard work necessary to restore public confidence.
It follows a review into the police's failure to hand over files on the 1992 UDA attack on a Belfast bookmakers in which five people were murdered.
The newly published report acknowledges that steps have been taken to increase staffing levels and transfer files to new IT systems since the 2018 blunder.
But it expressed fears that similar omissions are possible in future cases without an urgent review into how officers are appointed and trained to deal with sensitive legacy documents.
Chief Inspector Jacqui Durkin said loss confidence in both the PSNI and police ombudsman is damaging to the community.
"This has highlighted the importance of having trusted and effective working relations at all levels," she said.
She said: "This needs to be supported by an effective memorandum of understanding to guide and provide clarity for both organisations at all levels and ensure legal obligations are met."
"Also that historic information held by the PSNI is identified, retained and made available to support Police Ombudsman investigations," she said.