A data tape containing information from almost 13,000 electricity bills has been lost by Northern Ireland's energy supplier.
NIE Energy said the tape did not contain any banking details.
Old billing information, names and addresses are included on the record of payment demands produced on 10 August last year, NIE Energy said.
A spokesman for NIE Energy said that while they had not been able to locate the tape there was no evidence to suggest it had fallen into the wrong hands.
The information commissioner's office has been alerted following the loss by contractors for NIE Energy who are in charge of bill processing.
Northgate Information Solutions is contracted by NIE Energy to provide data management services including organising the backing up of billing information through its subcontractors, Liberata and Formscan.
Each day after electricity bills are produced Liberata transfers billing information by tape to Formscan to be backed up onto microfiche.
It was during this process relating to a single day's electricity bills in August 2009 that the tape went missing.
NIE Energy said Northgate had already overseen improvements to ensure this type of incident does not happen again.