Martin McGuinness personally set up the rendezvous which led to the brutal murder of a suspected IRA informer, the Dublin government was told in 1987.
Previously secret files in the Department of Foreign Affairs in Dublin reveal the then Bishop of Derry Edward Daly made the damning claim seven months after the killing of Frank Hegarty.
Bishop Daly said Mr McGuinness normally did not get his "hands dirty" but had run out of henchmen in the city.
Mr Hegarty, a Provo quartermaster in Derry, was abducted from Buncrana, Co Donegal, and shot in the head in May 1986 after he had been lured home with claims he would be safe.
His body was dumped on the side of a border road with his eyes taped.
A typed letter, marked secret, was filed to the Department of Foreign Affairs by an official who had met Bishop Eddie Daly and talked about the execution.
Released under the 30 year rule, it said: "The Bishop understands that, far from using a henchman (as he would ordinarily do), McGuinness personally arranged the rendezvous with Hegarty from which the latter did not return."
Bishop Daly said the former IRA commander turned peacemaker had been doing "reckless things" at the time.
He said these actions would make Mr McGuinness "vulnerable if he were to come under media scrutiny".
Over the years Mr McGuinness, who died last March, faced repeated questions over the Hegarty murder but always insisted he had "no role whatsoever".
The dead man's family have said the former Deputy First Minister persuaded Mr Hegarty to come home. And Bishop Daly believed them.
It is understood Mr Hegarty fled to England, protected by British intelligence, and is reported to have given information on a dump of IRA arms smuggled from Libya before being lured home.
Bishop Daly said Mr McGuinness assured relatives on a number of occasions that Mr Hegarty would not be harmed.
The Bishop was reported to have said: "McGuinness would usually try to 'keep his own hands clean' in an affairs of this sort but, with the number of Provo volunteers in Derry reduced... by rumours that Hegarty had 'squealed', McGuinness was left in a position for several months last year in which he had to do much of 'the dirty work' on his own."
Bishop Daly said he was certain Mr McGuinness was a Provisional IRA Chief of Staff "at least for the North-West if not for the entire North".
The letter was dated January 22 1987, about seven months after the murder.
It was sent to Dublin and copied to the Tanaiste and the Ambassador in London, as well as the secretary of the Dublin government's Anglo-Irish Secretariat.
It has been reported Mr McGuinness met Mr Hegarty's mother Rose on numerous occasions as he tried to coerce him to return home, including a claim he went down on bended knee.
A sister of Mr Hegarty is also said to have unwittingly driven him to the rendez-vous in Buncrana.
The documents can be read in the 2017/20/17 file from the Department of Foreign Affairs.
