The Northern Ireland Office strongly rebuffed an Irish government query about the murder of British soldier Stephen Restorick.
The soldier was killed by an IRA sniper while manning a checkpoint at Bessbrook in south Armagh in February 1997.
Declassified NIO files show that David Donoghue, Irish joint secretary of the Anglo-Irish Secretariat, asked his northern counterpart Peter Bell about the possibility that the soldier had been killed by a round caused by an accidental discharge.
Mr Donoghue claimed to have heard this story from "RUC sources".
Mr Bell wrote he had informed Mr Donoghue that, having consulted the military in the area, "this was emphatically not the army view".
He added: "As well as strongly encouraging Mr Donoghue not to give credence to these stories on the Irish side, I added that it would win the Irish no friends here if the idea got around that they were seeking to extenuate or explain what we believe was a straight IRA murder."
Following the murder, Secretary of State Sir Patrick Mayhew wrote to the UN Ambassador to the UK, Admiral William Crowe.
"Last night's cold-blooded murder of a young soldier, shot in the back by a sniper while manning a checkpoint in County Armagh, is the latest in a series of steadily intensifying terrorist attacks on the security forces... Nothing expresses more clearly the hypocrisy of Sinn Féin's talk of peace," Sir Patrick wrote.
"The latest murder has caused deep anger…and has been accompanied by widespread appeals to the loyalists to maintain their ceasefire."
