The former Northern Ireland Police Ombudsman Baroness Nuala O'Loan has claimed that "hundreds and hundreds" of deaths happened in Northern Ireland as a result of security force collusion.
Baroness O’Loan said that some paramilitary informants recruited by the British army and the RUC were “serial killers”.
Her claims were disputed by the PSNI chief constable George Hamilton.
Baroness O'Loan made her allegations on Thursday night's BBC Panorama programme, Britain's Secret Terror Deals, which examined the extent of security force collusion with republican and loyalist paramilitaries
She said that the police and British army allowed informers to commit crimes, up to and including murder, with “impunity”.
“They were running informants and they were using them. Their argument was that by so doing they were saving lives, but hundreds and hundreds and hundreds of people died because those people were not brought to justice and weren’t stopped in their tracks,” she said.
“Many of them were killers and some of them were serial killers.”
The programme dealt with killings such as that of the 1976 IRA Kingsmill Massacre of 10 Protestants, the 1982 UDA shooting of five Catholics in Sean Graham's bookies on the Ormeau Road in Belfast, the 1989 UDA murder of Belfast solicitor Pat Finucane, the 2001 Loyalist Volunteer Force murder of Sunday World journalist Martin O'Hagan and the IRA murder of RUC constable Colleen McMurray in Newry, Co Down, in 1992.
It also addressed some of the killings carried out by members of the so-called Glenanne Gang allegedly comprised of members of the UVF, RUC and British army.
They included the killings of 34 people in the 1974 Dublin-Monaghan bombings.
The programme also referred to how the current Police Ombudsman Dr Michael Maguire is investigating claims that both RUC and PSNI officers did not conduct proper murder inquiries into more than 60 murders because they were protecting informers or that there was police collusion in the murders.
Mr Hamilton queried Baroness O’Loan’s assessment of the “scale” of killings and said informers saved “thousands of lives”.
He conceded however that during the Troubles, there were “no rules” governing how security force handlers dealt with paramilitary agents.
He told the BBC, “My understanding is that there were hundreds if not thousands of lives saved through the work of informants and police and, in those days, (British) army working with those informants. I’m not saying that everything that was done was done to the standards of today.”