O2 has said a tape that may have contained customer information went missing from its office last year.
The mobile phone operator said the tape that possibly contained some information relating to its 1.3 million customers went missing in September 2011.
It said it only became aware of the incident earlier this summer when its IT partner IBM informed them about it.
O2 said the tape was used to back up daily work files and would have captured a snapshot of information held by the company’s internal system at that time.
It believes there is a low risk to customer data as most of the information on the tape related to internal company business.
O2 also said it believes the tape could still be in one of its offices and that it is in a format that is not easily accessible.
The Office of the Data Protection Commissioner said it is treating the incident seriously, but that as the tape was lost internally, customers should not be unduly concerned.
It has asked O2 to notify all 1.3 million of its customers as of September 2011 about the incident.
The Commissioner warned those customers to be cautious about any contact they receive that appears to be based on information only known to O2.
Its investigation into the data breach is ongoing.
In a statement on its website, O2 said it is the first incident of its kind and they have put a review in place to ensure it does not happen again.