The actions of a senior British informer in the Irish Republican Army (IRA) codenamed "Stakeknife" probably cost more lives than they saved in Northern Ireland's sectarian conflict, an independent inquiry found on Friday.

The nationalist IRA, which opposed British rule in the province, was responsible for more than half the 3,600 deaths during three decades of guerrilla conflict, which were largely ended by a peace accord in 1998.

Operation Kenova's interim report found that British security forces were frequently aware of imminent abductions and murders, yet failed to protect those at risk.

As a result, it said, murders that should have been prevented were allowed to take place, and those responsible were left free to reoffend.

"This report leaves little doubt that the Republican leadership was responsible for numerous dreadful crimes, many of which the government failed to prevent," former Kenova lead Jon Boutcher said.

Boutcher led the eight-year probe before becoming head of Northern Ireland's police last year. He presented the findings to reporters alongside his successor, former Scottish police chief Iain Livingstone.

The 212-page report did not reveal the identity of the agent alleged to have belonged to the IRA's feared "Nutting Squad", which interrogated suspected informers.

Freddie Scappaticci, a Belfast man who died last year, denied being "Stakeknife" but lost a legal bid to force British ministers to state that he had not been a double agent.

The report said it was well known that Scappaticci had been a member of the IRA's internal security unit. (Reuters)