An inquiry into the murder of Loyalist prisoner Billy Wright in the Maze Prison found there was no "state collusion" in his killing.
The Government commissioned a £30 million report after investigations by retired Canadian judge Peter Corry into allegations of collusion by prison staff in the 1997 killing.
Northern Ireland Secretary Owen Paterson told the House of Commons that there were "serious failings" by the prison authorities which did "facilitate" Wright's death but that they were result of "negligence" and were not intentional.
The leader of the Loyalist Volunteer Force splinter group, who was allegedly linked to up to 20 murders of mostly innocent Catholics, was sitting in the back of a prison van at the notorious Maze Prison waiting to be taken to meet his visiting girlfriend when he was shot seven times.
The murder, just two days after Christmas 1997, threatened to disrupt the tense all-party political negotiations in the months before the signing of the Good Friday Agreement the following year.
Wright, 37, was ambushed by three armed republican prisoners belonging to the Irish National Liberation Army, who managed to slip through security and open fire with a semi-automatic pistol and a double-barrelled .22 Derringer.
They surrendered themselves to prison staff and were later sentenced to life imprisonment but released early under the terms of the 1998 peace deal.
Two of the three had been transferred into the same H-Block as Wright the previous May, just weeks after Wright was moved from Maghaberry Prison, also near Lisburn, Co Antrim, to serve out an eight-year sentence.