The PSNI is under fire for failing to disclose "significant, sensitive information" about a loyalist massacre to the Police Ombudsman.
Sinn Fein last night said it was seeking an urgent meeting with Chief Constable George Hamilton to discuss the force's "appalling and unacceptable" failure.
Victims' groups also hit out at the PSNI, which has apologised for its actions.
Ombudsman Dr Michael Maguire has asked the Department of Justice for an independent review into how police disclose information.
The Relatives for Justice campaign group said this "should begin as a matter of urgency".
The Police Ombudsman revealed that the PSNI hadn't disclosed information, some of which related to covert policing, to its investigators working on the UFF's 1992 massacre at Sean Graham's bookies on Belfast's lower Ormeau Road.
Dr Maguire said the material's existence came to light when police prepared to disclose it to the bereaved families as part of civil proceedings.
The PSNI insisted it hadn't withheld the information deliberately and put the omission down to human error.
The ombudsman's office said the material in question had opened new lines of inquiry in its investigation into the bookies massacre in which five people died.
It also did the same regarding a range of loyalist killings in the north west between 1988 and 1994, and the ombudsman's investigation into the 1993 murder of teenager Damien Walsh at a west Belfast coal depot.
Reports outlining the findings of these investigations, which had been due to be published in coming weeks, will now be delayed.
In a significant step, the PSNI last night offered to now give ombudsman investigators "full and unfettered access" to police legacy systems. A spokesman for the ombudsman's office welcomed the move.
In a statement yesterday, Dr Maguire said: "My staff became aware that police were preparing to disclose a range of material as part of impending civil proceedings.
"Following a request from this office, police released this material to us which helped identify significant evidence relevant to a number of our investigations.
"Following on from this, police have now also identified a computer system which they say had not been properly searched when responding to previous requests for information.
"In that instance, it would seem information which police told us did not exist has now been found."
Dr Maguire said effective disclosure of information was vital in dealing with the past.
"The public must have confidence that, when asked, police provide all the relevant information they hold on given matters," he said. "The police have told us the problems came about through a combination of human error arising from a lack of knowledge and experience and the complex challenges associated with voluminous material - some 44m pieces of paper and microfilm records - that is stored in various places and on a range of media and on archaic IT systems."
Dr Maguire added: "In the interests of public confidence in policing, I have contacted the Department of Justice to ask that an independent review be carried out into the methods police use in disclosing information."
It is understood that the material has implications for up to 30 murders.
Sinn Fein MLA Gerry Kelly said: "The revelations that the police failed to disclose key information to investigations by the Police Ombudsman into dozens of killings by loyalist death squads is appalling and unacceptable.
"Many people will see this as further evidence that the cover-up of the role of British state forces in the conflict in Ireland is systemic.
"The PSNI may claim that it is an error but how long was it after the PSNI hierarchy knew about this failure before they informed the Police Ombudsman or the families affected?"
Mr Kelly said his party would be seeking an urgent meeting with George Hamilton and would also be raising the issue at the Policing Board.
"The PSNI or the British Government saying sorry is not enough. Confidence in policing is diving to its lowest level in many years. Immediate action is needed," he added.
Former Police Ombudsman Baroness Nuala O'Loan told the Belfast Telegraph that her heart went out to the families.
"They have been fighting for so long to try and get to the truth of what happened and now this," she said.
"The PSNI have had decades to get their files in order and still it has not been done.
"The question must be what else lies where they found this material? How many other cases are affected? How has this come to pass?"
Baroness O'Loan continued: "What the Chief Constable needs to do now is to do what was supposed to be done years ago - release all the Special Branch files and other material in compliance with the law.
"Those who suffered during the years of the Troubles and who still suffer, deserve the truth, not empty promises, not handwringing, not vacillation.
"The future of policing must not involve ongoing failings of this magnitude.
"It does not have to be like this. It is appalling."
The director of the Committee on the Administration of Justice, Brian Gormally, said the PSNI's claim of human error "simply insults our intelligence".