Four CDs containing details from court cases have gone missing in the post.
The computer disks vanished after being sent by recorded delivery.
Greater Manchester Police (GMP) had previously hand-delivered the four CDs to Her Majesty's Inspectorate of Court Administration (HMICA) last December.
The disks contained details of at least 55 magistrates' court defendants and other "restricted" data not released in open court, the Daily Mail reports.
A police spokesman could not confirm whether the disks contained details of witnesses.
HMICA has launched an internal investigation into what happened to the disks.