A new book claims members of the RUC and UDR were part of a loyalist gang that killed more than 100 people in the 1970s.
The book claims to have uncovered evidence of collusion on a huge scale.
It says the loyalist gang operated from farms in counties Armagh and Tyrone.
One extract, from an unpublished HET report, says there was "indisputable evidence of security forces collusion" that should have rung alarm bells all the way to the top of government.
Lethal Allies - British Collusion in Ireland contains other extracts of unpublished reports by the Historical Enquiries Team (HET) that refer to evidence of widespread collusion.
The book is written by a researcher with the Pat Finucane Centre, a human rights advocacy and lobbying group in Northern Ireland.
It is published on Thursday.
The author, Anne Cadwallader, claims RUC officers and members of the UDR were part of a gang operating from two farms in south Armagh and Tyrone that killed 120 people between 1972 and 1976.
"It can be demonstrated beyond a reasonable doubt that there was systemic collusion in these cases," she said.
Allegations of collusion between members of the security forces and loyalist paramilitaries are nothing new.
The BBC Spotlight programme investigated allegations about the existence of the gang nine years ago.
At the time, two former RUC officers admitted they had been members of the gang.
One of them was William McCaughey, who has since died. He was convicted of the murder of a Catholic shopkeeper and the kidnapping of a Catholic priest.
He told BBC Spotlight he had been a member of the UVF while serving as a police officer and explained how it operated.
"There was horses for courses obviously," he said.
"There was people who were drivers, there was people who were able to take people's lives, shoot people. There was people who simply operated on an intelligence level, not every person involved was actually involved in the taking of life directly.
"I was quite happy to co-operate at any level with any person who had a basic loyalty to Northern Ireland, who shared the same belief system, the same ideological system as myself."
The book claims the loyalist gang operated from farms in Glennane in south Armagh, and Dungannon, County Tyrone, and that the security forces were aware of their activities.
An extract, referring to the killings of four people in attacks on two pubs in Charlemont, County Armagh, on a night in May 1976, says: "It is difficult to believe... when judged in concert with other cases emerging at the time, that such widespread evidence of collusion in these areas was not a significant concern at the highest levels of the security forces and of government."
Another HET report also claims that the RUC had advance knowledge of an attack in which two people died in August 1976.
The book claims that days before the attack on the Three Steps Inn in Keady, County Armagh, the RUC knew a bomb was being stored at a farmhouse owned by a serving police officer and asked the army to put it under surveillance.
According to the book, the surveillance operation was ended and the bomb was then used in the attack.
It also claims that RUC Special Branch knew the identities of four people involved in the bombing, but that no arrests were made.
The families of the two victims, 38-year-old Elizabeth McDonald and Gerard McGleenan, 22, have lodged complaints with the Police Ombudsman.
A former senior member of the RUC Special Branch said he was ashamed that members of the force were involved in collusion.
"Obviously it presents a very damning picture and there's nobody more ashamed than myself and my colleagues," Raymond White said.
"These people were serving in and amongst other officers who had taken the oath of allegiance to uphold the rule of law and this small cadre of individuals were actually there breaking the law."