The PSNI is facing legal action from an independent watchdog that accuses it of refusing to provide information about more than 60 deaths.
Police Ombudsman Michael Maguire has served notice on the PSNI over its alleged failure to provide his investigators with material and said his inquiries had been stalled.
The ombudsman's office is part of an elaborate scrutiny mechanism overseeing policing following the peace process.
It is tasked with investigating complaints about police conduct surrounding historical and more recent inquiries.
Its judicial review will seek to compel the PSNI to provide the ombudsman with information.
Dr Maguire said: "We cannot have a situation where any public body, and particularly the police, can decide whether or not it will cooperate with a criminal or misconduct investigation, particularly where legislation requires them to do so."
The ombudsman handles complaints about the conduct of officers.
It has undertaken a series of hard-hitting investigations that found serious faults, although in many other cases it exonerated detectives.
Association of Chief Police Officers (Acpo) Chief Constable Hugh Orde, who formerly headed the PSNI, has called for a similar regulatory system to be introduced across the UK.
Dr Maguire said despite repeated requests over past months, the PSNI has on more than 100 occasions either refused to provide information to his office or has said it must first explain and justify why the material is wanted.
He said: "The police have taken the view that they will decide whether or not to provide us with information and in many cases have now decided not to."
He said the legal action was unusual and unfortunate but necessary, adding: "The many thousands of people who make complaints to us every year do so on the basis that we have access to all the police information we need to independently investigate their complaint.
"That principle is enshrined in law and accepted across the community. Investigation by negotiation is not acceptable."
A PSNI spokesman said the organisation has a legal responsibility for the care and management of all information that it holds, a responsibility which must be taken extremely seriously.
"At the same time the PSNI also recognises the statutory responsibility to provide information to the Police Ombudsman, enabling exercise of his functions and legal responsibilities.
"Police are currently seeking to agree a solution with PONI around these complicated, and sometimes unfortunately competing, legal issues.
"We will continue to fulfil our legal obligations with the primary consideration being that of protecting life in accordance with Article 2 of the European Convention on Human Rights."
