RUC agents escaped prosecution for at least a dozen murders because they were protected as informers, Police Ombudsman Nuala O'Loan is expected to confirm.
A long-awaited report, due to be published on Monday, is understood to provide damning evidence that Special Branch effectively allowed a UVF gang operating on the Mount Vernon estate in north Belfast to murder with impunity because more than half were police agents.
While previous investigations by Lord Stevens and retired Canadian judge Peter Cory have highlighted individual cases of security-force collusion with loyalists, Mrs O'Loan's report is expected to confirm that police agents were permitted to carry out at least 12 murders between 1993 and 2002 without fear of prosecution. Some reports suggest the number could be as high as 24.
She is expected to deliver a 200-page report to Secretary of State Peter Hain, Chief Constable Sir Hugh Orde and the Policing Board but only 100 pages will be made public.
Mrs O'Loan was originally asked to investigate the claim that the UVF's Mount Vernon leader Mark Haddock was being protected as a police informer but her inquiries are understood to have expanded quickly as she uncovered evidence that Special Branch had blocked legitimate police inquiries into a series of murders because Haddock and other agents were implicated.
A key aspect of her report is understood to relate to the actions of at least six Special Branch officers within the RUC's D Division in north Belfast. These handlers are alleged to have protected or failed to take action against at least six informers within Mount Vernon UVF despite evidence of their involvement in a string of murders.
One of the most damning allegations is that Special Branch officers not only thwarted the arrest of agents but also tipped off the killers about police investigations into their activities.
It is being speculated that Mrs O'Loan's report might even go further and ask whether any high-ranking RUC officers were made aware of the involvement of police agents in murder.
Last September Mr Hain admitted that the report would provide "extremely uncomfortable" conclusions when it was eventually made public.
Victims' relatives are waiting to hear whether Mrs O'Loan has recommended charges against any Special Branch officers, who are all understood to have retired in recent years. She provided the Public Prosecution Service (PPS) with an interim report of her findings in 2004.
It is understood that at least one victim's family are keen to ask exactly when Sir Hugh became aware of Haddock's role as an informer.
Former RUC detective Trevor McIlwrath is understood to have first claimed in May 1999 that Haddock was being protected as a Special Branch agent despite being involved in multiple murders.
His allegation came while he and colleague Johnston Brown were assisting Lord Stevens's inquiry into the murder of Catholic solicitor Pat Finucane. Stevens inquiry detectives are understood to have told Mr McIlwrath and Mr Brown that the remit of their inquiry did not allow them to investigate the allegations made against Haddock.
In December that year Sir Hugh took over day-to-day control of the Finucane inquiry from Lord Stevens.
The family of at least one victim are understood to have already asked in writing whether Sir Hugh was made aware of Haddock's role as an informer at that time.
Haddock is alleged to have been involved in at least two murders after December 1999.
Sir Hugh was appointed as PSNI chief constable in May 2002 and immediately implemented changes which included the merger of Special Branch and CID into a Crime Operations Department.
Last June the PSNI announced that it had 'deactivated' a quarter of its informers after a two-year review of 'covert human intelligence' sources. Haddock is understood to have been one of the first to be 'deactivated'.
In an ironic twist in 2004 a planned meeting between Mr Brown and the ombudsman's detectives had to be cancelled after it emerged that Haddock and his Special Branch handlers were meeting in the same hotel on the outskirts of south Belfast.
Mr Brown and Mr McIlwrath have said they fear that Haddock's handlers will not be prosecuted and that they could be used as scapegoats.
Last August the pair were arrested by the ombudsman's detectives and questioned over alleged misconduct of office relating to a failure to charge Haddock with the murder of Catholic taxi driver Sharon McKenna in January 1993.
The two detectives are understood to have stated that Special Branch and senior RUC officers blocked them from charging Haddock with the murder.
Questions have also been asked about whether any Special Branch officers or agents will face charges even if Mrs O'Loan recommends this course of action.
Sceptics point to the fact that the PPS has failed to recommend a single charge against any of the 20 security-force members Lord Stevens accused of collusion in April 2003.