Image copyright Pacemaker Image caption Fred Scappaticci strenuously denies he was an Army agent within the IRA

Stakeknife, the top British spy within the IRA, was a key factor in successful Army operations against the group in County Tyrone, a relative of one of those killed has told the BBC.

Twenty-six IRA men based in the county were shot dead by the SAS during the Troubles.

BBC Spotlight examines the role of agents in the latest part of its Secret History series.

West Belfast man Freddie Scappaticci has denied being Stakeknife.

The agent, who was working for the Army, headed up the internal IRA investigation into the Loughgall ambush in 1987.

The SAS, the elite Army unit, was lying in wait for an eight-man IRA team as it attacked a police station, and shot them dead.

The investigation did not find out who was responsible for compromising the operation.

The programme, quoting republican sources, states a local IRA man, Gerard Harte, fell out with Stakeknife over who may have been to blame.

Image copyright Pacemaker

Mr Harte was later killed in another SAS ambush near Drumnakilly in 1988.

His brother, Ignatius Harte, was asked by BBC Spotlight if he held Stakeknife responsible.

"If Freddie Scappaticci was dealing with internal (IRA) security in Tyrone, which we know he was, obviously that was a leading role in how so many operations were carried out in Tyrone.

"All wars are dirty wars, but this was an exceptionally dirty war."

Kieran Conway, a former IRA intelligence officer, told the programme: "The attrition rate was just so appalling.

"British intelligence were obviously in a position to intercept most operations."

Who is Stakeknife?

Image caption Mr Scappaticci left Northern Ireland when identified by the media as Stakeknife

Fred Scappaticci is alleged to have been the most high-ranking British agent within the Provisional IRA, who was given the codename Stakeknife.

He was the grandson of an Italian immigrant who came to Northern Ireland in search of work.

He has admitted to being a republican but denied claims he was an IRA informer.

He is believed to have led the IRA's internal security unit, known as 'the nutting squad', which was responsible for identifying and interrogating suspected informers.

Mr Scappaticci left Northern Ireland when identified by the media as Stakeknife in 2003.

The activities of Stakeknife - for decades the Army's "golden egg" within the IRA - are being investigated by the former chief constable of Bedfordshire, Jon Boutcher.

His inquiry, Operation Kenova, could involve about 50 killings.

Mr Boutcher has previously said he may be able to bring charges against former members of the IRA, the Army and MI5.

You can see the fourth episode of Spotlight on the Troubles: A Secret History on BBC iPlayer