PSNI Chief Constable Matt Baggott is to face tough questioning today from the Policing Board over his refusal to hand over intelligence material to outside investigators.
On Tuesday Police Ombudsman Michael Maguire said he is taking legal action to obtain sensitive material on informants and agents.
He wants the intelligence on 60 murders and other incidents where there are allegations of criminal behaviour or misconduct by PSNI officers in public office.
The police have said they believe they have responded appropriately to each request for information.
Mr Maguire said he had no option but to take legal action because he had received more than 100 refusals for information.
He said the material was needed for his investigators to do their jobs.
Mr Maguire claimed the chief constable was making it impossible to investigate allegations of serious criminal activity and misconduct.
The PSNI said it was seeking to agree a solution with the ombudsman about what it called “complicated and, sometimes, competing legal issues”.
When news of the legal action broke, the Policing Board said in a statement: “Police cooperation and the provision of information to the institutions with legislative responsibility for delivering independent oversight and accountability of the PSNI is critical.
“The application for leave for judicial review by the Police Ombudsman is very significant and a matter of great concern.”
One of the cases relates to the Loughinisland massacre in June 1994 when the UVF shot dead six men in the Heights Bar as they watched a Republic of Ireland World Cup match.