In 1993, a year before the IRA first laid down its arms, the existence of a secret “back channel” between Martin McGuinness and MI6 was revealed on the front page of the Observer.

The so-called Derry Link – which connected the British government with the Provisionals’ leadership via a former priest – eventually led to the IRA’s cessation of violence a year later.

At the time the revelation of a clandestine nexus was sensational. But a new book on the outbreak of the Troubles 50 years ago this month shows that it had in fact existed long before, and had been used to stop specific IRA attacks even during the very earliest years of the conflict.

Denis Bradley, a cleric who more than 20 years later organised covert talks between McGuinness and MI6 officers, passed on advice from Derry’s chief police officer that the Provisionals should call off certain shootings and bombings in the early 1970s, according to the book.

Facebook Twitter Pinterest The Observer front page 28 November 1993, which revealed the secret backchannel between MI6 and the IRA. Photograph: The Observer

In Fifty Years On by the Belfast journalist and author Malachi O’Doherty, Bradley says he and a Royal Ulster Constabulary veteran, Frank Lagan, convinced the local IRA to cancel some operations as they knew they had been compromised in advance by special branch via its informers.

Reflecting on his relationship with Lagan, Bradley says: “He and I became fairly, well, ‘close’ – a strange word because Lagan was a very tall, austere human being, not the kind of guy you could have a close emotional contact with, but I was very trusting of him and he was trusting of me.”

Bradley, who was ordained in 1970 but gave up the priesthood at the end of that decade, was also trusted by McGuinness, then a rising star in the emerging Provisional IRA in Derry and beyondT who went on to become Sinn Féin’s chief negotiator in the peace process. Lagan thought he could use his contacts to stymie some attacks.

It was not straightforward: in the book Bradley claims Lagan told him that RUC special branch knew about some upcoming IRA operations but let some go ahead – probably because it would protect informers inside the Provisional’s Derry Brigade. Nonetheless, Bradley says, he and Lagan together sent messages to the IRA leadership in the city that halted a number of attacks. He refuses to specify which IRA operations he and Lagan allegedly helped stop.

“I also was aware through him [Lagan] of the difficulty with Special Branch because he was informing me quite often of how difficult it was. He used me on a number of occasions to get things stopped because special branch would have let it go ahead and people would have ended up dead,” he says.

Bradley went on to become vice-chairman of the Northern Ireland Policing Board in the early 2000s. Lagan was the RUC’s chief superintendent in Derry at the time of the Bloody Sunday march in 1972.

He proposed marchers be allowed through to their planned destination in Derry’s city centre, and later argued the violence which left 13 unarmed civilian protesters dead – could have been “relatively contained” if that had happened. Through his link with Bradley, Lagan was given assurances from the Provisional and Official IRA that there would be no guns at the protest. Lagan died in 2005 aged 88.

• This article was amended on 12 August 2019 to correct the year of Lagan’s death.