Police Ombudsman Nuala O'Loan concluded that former officers in the secretive Special Branch paid informants in the outlawed Ulster Volunteer Force who were permitted to pursue killings, bombings, drug dealing and extortion.
Her 162-page report called for police to reopen dozens of cases from the 1990s and investigate ex-officers involved in cover-ups of their informants' crimes.
The commander of the predominantly Protestant police force, Chief Constable Hugh Orde, said he accepted O'Loan's conclusions and recommendations in full and offered "a wholehearted apology for anything done or left undone."
In the report, both Orde and O'Loan noted that the police force's intelligence-gathering arm had been drastically reformed since 2003 in hopes of ensuring such abuses never happened again.
In London, the official spokesman for Prime Minister Tony Blair called it "a deeply disturbing report about events totally wrong that should never have happened. The fact that they did is a matter of profound regret and the prime minister shares that regret."
Blair's secretary of state for Northern Ireland, Peter Hain, said UVF veterans and former police officers both should stand trial for crimes.
"There are all sorts of opportunities for prosecutions to follow. The fact that some retired police officers obstructed the investigation and refused to cooperate with the police ombudsman is very serious in itself," Hain said.
O'Loan, a Catholic with the power to investigate misconduct in Northern Ireland's police, was scheduled to hold a Belfast press conference later Monday.
Her report found that Special Branch agents covered up for Protestant extremists in an Ulster Volunteer Force unit based in Mount Vernon, a hard-line Protestant neighborhood in north Belfast, throughout the 1990s.
Her investigation started with the 1997 killing by that UVF unit of a 22-year-old Protestant man, Raymond McCord Jr., who had been a member of the paramilitary group.
The victim's father, Raymond McCord, said he had evidence that the UVF unit's commander at the time, Mark Haddock, was protected by police because he was on the Special Branch payroll providing tipoffs on UVF activities.
McCord said he turned to O'Loan after senior police officers dismissed him as "some sort of crank."
O'Loan's report concluded that Special Branch detectives turned a blind eye to several killings and other crimes committed by Haddock's unit because of the information they were receiving from him and other, lower-ranking UVF members.
The published report did not identify by name any of the retired Special Branch officers involved in collusion. A secret version of the report that includes these names was delivered over the weekend to Orde, Hain and a handful of other British officials.
One of the former detectives arrested and questioned by O'Loan's investigators, Johnston Brown, said many rank-and-file detectives were prevented from doing their jobs by a Special Branch elite that hoarded information.
Brown, who was a detective in the police's Criminal Investigations Division, said Special Branch colleagues repeatedly stymied his efforts to solve crimes involving members of the UVF and another outlawed Protestant group, the Ulster Defense Association.
In a statement, a group of former Special Branch officers rejected the findings. The ex-officers said they "always acted in the best pursuit of justice and had nothing to be ashamed of."Reuse content