LONDON, JAN. 25 -- Allegations that police in Northern Ireland operated a "shoot-to-kill" policy against presumed terrorists during 1982 have been dismissed by Britain's attorney general, who said today that there would be no further prosecutions in the deaths of five unarmed terrorists and a teenager shot by police that year.

Attorney General Patrick Mayhew's announcement in the House of Commons followed a three-year investigation that began in 1984, when Northern Ireland politicians charged that earlier police acquittals in the cases had been the result of an official cover-up.

The announcement, which had been awaited since the investigation report was turned over to government prosecutors last spring, caused an uproar in the House. Ken Livingstone, a member of the opposition Labor Party, was expelled from the Commons for the rest of the week after accusing Mayhew of being an "accomplice to murder."

Kevin MacNamara, Labor's official spokesman on Northern Ireland, called the government's decision not to prosecute "insufferable."

The decision is likely to increase tension between Britain and the Republic of Ireland, who two years ago agreed to form a bilateral commission to work on alleviating violence in Northern Ireland. The Irish government has complained in recent months that Britain has been dragging its feet on a part of the agreement calling for reform of judicial procedures that Dublin says discriminate against minority Catholics in the province.

In a statement tonight, the Irish government expressed its "deep dismay and concern" over the British announcement and said it would seek "urgent clarification" of Mayhew's statement.

Five of the dead men were acknowledged members of terrorist organizations, although none was carrying a weapon or engaged in paramilitary activity at the time of the police shootings. The sixth was a teen-ager, uninvolved in Northern Ireland violence and apparently shot by mistake.

Mayhew said there was no evidence of "any offense, such as incitement to murder, such as would be comprised in a shoot-to-kill policy." He said further prosecutions would not be in the "national interest."

The complex events leading to today's announcement began in mid-1982, when the Royal Ulster Constabulary, the province's police force, was battling a heavy IRA terrorism offensive in which a number of officers had lost their lives.

In the fall, police fired 109 rounds of ammunition into a car that allegedly had failed to stop at a checkpoint in County Armagh, which shares a border with the Irish Republic. Three unarmed IRA members inside the car were killed. Three constabulary men later stood trial for one of the deaths and were acquitted.

The second shooting took place in November 1982 when two teen-age boys, riding their bicycles through the countryside, stopped and entered an old shack that was under police surveillance as a possible IRA arms dump. Police fired on the boys, killing one and seriously wounding the other, allegedly without proper warning. No one was ever prosecuted in the incident.

The third incident was in December 1982 when police shot two unarmed men, subsequently identified as members of an IRA splinter group, after their car allegedly failed to stop at a roadblock. A constabulary officer later stood trial for murder and was acquitted. But it emerged in court that senior officers had instructed the defendent to perjure himself about the circumstances at the time of the deaths and afterward.