Police colluded with loyalists behind several murders in north Belfast, a report by the Police Ombudsman of Northern Ireland is to confirm.
The investigation was sparked by the murder of Raymond McCord jnr
Nuala O'Loan's report will say UVF members in the area committed murders and other serious crimes while working as informers for Special Branch.
The report will also say some Special Branch officers protected the killers and ensured they were not caught.
NI Secretary Peter Hain said it "shone a torch into a very dark corner".
The report, to be published on Monday, will call for a number of murder investigations to be re-opened.
But it is unlikely that any of the police officers involved will be prosecuted - the ombudsman is expected to say that evidence was deliberately destroyed to ensure there could not be prosecutions.
The ombudsman's investigation began more than three years ago when Belfast welder Raymond McCord claimed that his son, also called Raymond, had been killed by a police informer.
The former RAF man, 22, was beaten to death and his body dumped in a quarry in 1997.
Sharon McKenna was murdered in 1992
Mr McCord has said he wants those who murdered his son to be put in prison.
He said he had received a death threat at the weekend from the UVF.
Mr McCord said that during his campaign of justice for his son he had been made to feel by police that he was "some sort of crank".
"They just passed me off as if it meant nothing," he said.
Among the investigations which could be re-opened are the murder in north Belfast in 1992 of 27-year-old taxi driver Sharon McKenna, who was shot at the home of an elderly friend.
The names of the police officers and the informers will not be made public.
Mr McCord was beaten and his body was left at a quarry
However, it is known that the main informer at the centre of the investigation is Mark Haddock, who was named in the Irish parliament 15 months ago as a UVF killer.
Mr Hain said the report had shocked him deeply and it would make "extremely uncomfortable reading", but that policing had changed.
"These things - murder, collusion, cover-up, obstruction of investigations - could not happen today, not least because of the accountability mechanisms that have been put in place over recent years," he told the BBC on Monday.
Some of the Special Branch officers criticised in the report have rejected the ombudsman's allegations as "unfounded and incapable of substantiation".
In a statement, the Northern Ireland Retired Police Officers' Association said they had always acted in the best interests of the pursuit of justice and had nothing to be ashamed of.
Mr Hain said the potential for charges was a matter for the PPS
The officers also challenged the ombudsman to disclose the details of any evidence of their criminal behaviour discovered during her investigation.
Johnston Brown, a retired detective sergeant in the RUC was questioned as part of the ombudsman's enquiry, but was not charged.
Mr Brown said Special Branch prevented him from carrying out proper investigations into a range of crimes including murder.
He said Mrs O'Loan's report could not possibly uncover the whole truth.
"I believe this is not the proper vehicle. Mrs O'Loan is going to make a number of recommendations based on very little facts.
I mean I was there on the ground suffering this obstruction and if you are not in possession of all the facts it's ill advised to lump the whole of the Royal Ulster Constabulary into the same pot."