The Independent Monitoring Commission (IMC), in a report released in Dublin and Belfast on Thursday, also concluded that some senior members of Sinn Fein, the IRA's political wing, had been involved in sanctioning the bank robberies.
"Some of its senior members who are also senior members of the Provisional IRA were involved in sanctioning the series of robberies," it said, without naming names.
The IMC recommended that the British government consider financial sanctions against Sinn Fein, the biggest Roman Catholic political party in Northern Ireland, including cuts in pay and benefits for Sinn Fein legislators.
London and Dublin both accept the conclusion of police chiefs in Northern Ireland and the Irish Republic that the IRA was behind the £26.5 million ($50 million) Northern Bank robbery.
The robbery, on 20 December, is believed to be the biggest heist in British criminal history. It also put into doubt the prospects of restoring power-sharing government in Northern Ireland.
Besides the Northern Bank robbery, the IMC report looked into three other robberies in May, September and October last year.