The data included prisoner records, policy notes and medical logs and was disposed of before the January 2005 law making public bodies more transparent.

The Northern Ireland Prison Service has been criticised for destroying security files on hundreds of terrorist prisoners held at the Maze.

SDLP Justice spokesman Alban Maginnis said: "It seems to me almost in contempt of the FOI Act and it is an extraordinary way to go about dealing with the new dispensation in relation to accessing documentation."

"It shows a very narrow and secretive attitude amongst the prison authorities and obviously it is regrettable that they stooped to such an excessive measure such as destroying a vast number of files."

An inquiry into the 1997 murder of LVF leader Billy Wright in the Maze Prison heard in November how 800 files with security information on terrorist prisoners released under the Good Friday Agreement had been shredded.

Wright, 37, was shot dead by three INLA gunmen on December 27, 1997, and Lord MacLean's inquiry is probing how the killers were able to target him in the high security centre.

Some of the material disposed of is uncontroversial and relates to medical and dental records. It is governed by a destruction timetable outlining the period which files have to be kept for.