But at another point, according to the Powell memoirs, the backchannel went completely dead, at least for a period.

In the post-Leeds Castle contacts, Powell relates in his new book, Great Hatred, Little Room that: "McGuinness continued to attach importance to his backchannel contacts with the DUP, and he dropped into No 10 after one meeting and told me he had shown them some potential IRA language for the statement and they had responded that it was in the ballpark.

"I said I would believe it when I heard if from the DUP myself," Mr Powell notes.

"McGuinness rang me later to say they were sorting all the remaining issues out with the DUP direct and did not need my services."

In Belfast later on for a meeting with Sinn Fein, Mr Powell recounts they arrived late "because they had been meeting with the DUP backchannel.

"When they arrived they handed us a draft IRA statement and told us if we showed it to the DUP they would welcome it. This again proved optimistic."

Though DUP deputy leader Peter Robinson and Gregory Campbell MP, among others, have denied any existence of a backchannel, Mr Powell claims it was established after the 2003 Assembly election when the DUP became the leading unionist party.

"They (the DUP) were no different from the British government at the time of John Major or Margaret Thatcher saying they never had contacts with the IRA - but actually [they were] doing so as well," his book said.

"It did play an important role in making possible that extraordinary meeting (in March last year) between Ian Paisley and Gerry Adams.

"They had never met, they had never spoken until they sat down for that photo-opportunity. . . if you hadn't had that backchannel building confidence over time, it would have been difficult."