A former RUC Special Branch officer has claimed the undercover unit saved the lives of up to 16,500 people during the Troubles through republican agents.
William Matchett also believes that they disrupted 85 per cent of the IRA’s operations.
The retired officer served with Special Branch during his time in the RUC and later with the PSNI’s equivalent.
While in the past former members have written about their experiences, it is rare for them to use their own name.
Secret Victory: The Intelligence War that Beat the IRA has been adapted from a Phd written by Dr Matchett and draws on research including interviews with other ex-Special Branch members.
As well as dealing with the background to the 1987 Loughgall ambush that resulted in the deaths of eight IRA men and a civilian, the book also discusses the use of agents within paramilitary organisations – which he refers to as “insurgents”.
Dr Matchett joined the RUC in 1982 and transferred to Special Branch in 1989. After a career break he left the force in 2014.
Deeply mistrusted by some nationalists, Special Branch has in the past been accused of colluding with loyalist paramilitaries.
Speaking to The Irish News, Dr Matchett said agents within republican groups saved up to 16,500 lives and insisted the number “is not an inflated estimate”.
He claimed that at any one time up to 15 “well placed” agents may have been active within the IRA.
Arguing that each agent saved 37 lives, that amounts to more than 500 annually during the three decades of the Troubles.
In the book Dr Matchett also describes agents as a “force multiplier, inducing paranoia within the IRA and loyalist terrorist organisations that prevented many attacks”.
He maintains that their activities resulted in the prevention of 85 per cent of planned attacks, saying they were "preventing murders almost as fast as the Provos were planning them”.
Asked if there are any estimates for the number of people who died as a result of Special Branch actions, either through negligence or deliberate action, the former officer said it was difficult to put a figure on it
“I don’t know how you would actually quantify it," he said.
The author, who makes a distinction between agents and informers, confirmed that during his career he handled both loyalists and republicans.
In the book he reveals that during the Troubles 60 per cent of all intelligence was from human sources, with 20 per cent obtained through bugging phones, buildings and cars.
Around 15 per cent of information was gathered through surveillance, with the remaining five per cent harvested from scouring through newspapers and community and even parish bulletins.
He said that while there is a perception in some quarters that informers are “the worst of the worst”, those who provided information had different motivations.
While some people “gave information without taking money”, full members of the IRA, described as being “green booked”, all received payment.
“I think by and large what came out and will probably be strange to most people was there was a degree of decency in these people,” he said.
“A lot of volunteers got involved in the IRA and it was ‘Yes, this is the cause, unite Ireland, kick the Brits out’. All of a sudden they see the result of their handy work and they go, 'I didn’t sign up for this'.
“There’s that element of decency. Your difficulty is that within an organisation you are trying to look at who are the most decent to get the hardliners - if the hardliners are driving it, how do we catch the hardliners.”
Dr Matchett said during his time in the police none of the informers he handled were ever exposed and killed by members of their own organisations.
He also said no information gathered by him ever resulted in security force operations where people were killed, and he found no evidence in his research of Special Branch actually being involved in attempts to kill people.
“I am not saying that the police response was perfect but I’m saying is there was nothing that I saw which turned around and said this person can die in order to protect A, B, C and D," he said.
“That was jumping out.”
Mr Matchett said he also found no evidence that “there was a policy coming out where they were working with loyalist death squads or anything like that”.
He said while he was not aware of it at the time, as the Troubles neared their end state agencies were backing republicans “into a corner but you were giving them a political exit, that was the strategy from very early on”.
He also claims that the IRA was “decimated” by the time it called off its campaign and that its “last functioning brigade was south Armagh”.