The Northern Ireland Secretary, Dr John Reid, has appointed Sir John Chilcott to review the issues arising from Sunday's break-in at the Special Branch office at the Castlereagh police complex.
Sir John was Permanent Under Secretary of State at the Northern Ireland Office from 1990 to 1997 and in 1999 was appointed by the Prime Minister, Mr Blair, as Staff Counsellor for the Security and Intelligence Services - MI5, MI6 and GCHQ - in which capacity he works part-time in the Cabinet Office in London.
Sir John will be assisted by Mr Colin Smith, a former Inspector of Constabulary and Chief Constable of Thames Valley, who will act as an assessor. Their inquiry will proceed in parallel with the criminal investigation already established by the Chief Constable, Sir Ronnie Flanagan.
Its terms of reference are to establish how unauthorised access was gained to a Special Branch office; the extent of any damage caused to national security; the adequacy of action subsequently taken to mitigate any such damage and to prevent unauthorised access there and in similar buildings elsewhere in Northern Ireland; and any wider lessons to be learned.
In the House of Commons yesterday, Dr Reid said: "The work undertaken by the Special Branch of the Police Service of Northern Ireland bears directly on the safety of lives, both in Northern Ireland and throughout the UK more generally. I am determined therefore that we establish the facts surrounding this serious incident as quickly as possible and ensure that all necessary remedial action is being taken."
Dr Reid added: "In the context of a live criminal investigation it is not helpful to put in the public domain information that would otherwise only be known by the perpetrators or the investigators. Suffice to say that all lines of inquiry remain open. As to the nature of the documentation, the Chief Constable has established a high-level team to assess any impact its theft could potentially cause."
He said it was unlikely Sir John's report could be placed in the public domain, but he would make a further statement to Parliament once it was received.
Sir Ronnie Flanagan has said the highly sensitive intelligence documents stolen in Belfast do not include informers' names. However, he did not say the files did not contain other details on informers.
Sir Ronnie said he was "baffled" by the incident and did not believe, at this stage, that lives had been endangered. It is understood many informers are highly nervous, and officers have spoken to most of their contacts. Sir Ronnie was speaking after a meeting with the Policing Board.
The office, known as Room 220, had moved to that location only a week ago due to refurbishment work within the base. This pointed very strongly to an "inside job", security sources said. The office houses the "source handling unit", which is the link point between Special Branch and their informers. It is understood the documents stolen contained information about contacts between Special Branch officers and their agents, including code names, code words, telephone numbers, dates, times and locations.
Former security force agent Mr Willie Carlin said he believed the documents had been stolen to be made public, not destroyed. Former agents were "horrified" at the theft, he said.
The most senior detective in Belfast, Chief Supt Phil Wright, is in charge of the police inquiry into the incident.