A former RUC officer has alleged that British intelligence tried to persuade the Ulster Volunteer Force to attack a Catholic primary school in Co Armagh in retaliation for the Kingsmill massacre, in a new documentary to be premiered in Belfast on Thursday.

In the feature length documentary, Unquiet Graves: The Story of the Glenanne Gang, ex-RUC officer John Weir claims that British military intelligence was behind a plot to attack the primary school at Belleeks in Co Armagh and to kill children and teachers.

Weir, a self-confessed member of the Glennane Gang, who was convicted of the 1977 murder of Catholic man William Strathearn, said the UVF was urged to carry out the attack in retaliation for the 1976 Kingsmill massacre in which the IRA singled out and killed 10 Protestant workmen.

“The plan that was decided on was to shoot up a school in Belleeks,” said Weir, who added that the targets would be “children and teachers”.

Long since released and now living in South Africa, Weir told the director of the film Seán Murray that the alleged plan by British military intelligence was to cause the situation in Northern Ireland to “spiral out of control”.

Mr Murray, who comes from a west Belfast republican family, said from his conversations with Weir in South Africa the alleged British military intelligence plot was to foment a “civil war”.

In the end the UVF refused to carry out the attack, said Weir. Mr Murray said that Weir in granting the interview was engaging in a “cathartic process – I think he was trying to get as much information off his chest as possible”.

The film is based on Anne Cadwallader’s book, Lethal Allies which recounts how the Glennane Gang of loyalist paramilitaries, frequently working in collusion with RUC officers and Ulster Defence Regiment soldiers in the mid-Ulster area, is estimated to have killed more than 120 people, the vast majority of them Catholics, between 1972 and 1976.

Lethal Allies is largely based on investigations carried out by the now defunct Historical Enquiries Team, a division of the PSNI, and on declassified papers and official reports.

The gang’s victims included the 33 people killed in the 1974 Dublin and Monaghan bombings, those killed in the 1975 gun and bomb attack on the Miami Showband, the 1976 killings of six members of the Reavey and O’Dowd families in south Armagh and the killings in August 1975 of Seán Farmer and 22-year-old Colm McCartney, a cousin of the late Nobel laureate Seamus Heaney.

Mr Murray (42) is a nephew and godchild of Danny McCann, one of three IRA members killed by the SAS in Gibraltar in 1988 where they were allegedly planning a car bomb attack on a British army changing of the guard ceremony. He is a son of the senior Belfast republican Seán “Spike” Murray.

Mr Murray, who initially ran a retail business but studied film-making in the past eight years, said those memories prompted him to get involved in documentaries that had an “over-arching look at collusion”.

He said that Unquiet Graves would be shown in independent cinemas in Ireland and will be broadcast by RTÉ in May. It will also be shown in the US, Canada, Europe and Australia and on iTunes, Google Play and Amazon Prime.

Mr Murray said the story of the Glennane Gang and its victims would be particularly revelatory for international audiences. “A lot of this local people will be well aware of it but internationally I think it is going to dumbfound audiences,” he said.

“Collusion has left a dark and terrible stain on the North of Ireland, the pain that’s been caused to thousands of people here is incredible. If there is ever going to be a healing process on this island, if we’re ever going to move forward in reconciliation, people need to be able to tell their stories, but more importantly we need truth from the state about their role in the conflict.”

The “not-for-profit” film is narrated by actor Stephen Rea and funded by charitable donations, crowd funding and by the film-maker.