O2 and its IT partner in Ireland, IBM, have mislaid a backup tape that may contain customers' data.
In a Wednesday statement on O2 Ireland's website, the mobile operator said IBM had told it about the loss during the summer of this year. The tape actually went missing more than a year ago, in September 2011.
Once it knew about the incident, O2 told the Irish Data Protection Commissioner (DPC), who told the company to inform its customers.
It appears that the tape was part of a set used for "daily backups of O2's systems", meaning it may contain some personal data, but is most likely to contain corporate data. It may also have been "misplaced within an otherwise secure location in O2", the telco said.
It remains unclear how many customers' details might have been on the tape, which O2 said was unencrypted. However, the company said it had around 1.3 million customers in Ireland at the time of the loss.
"We believe there is a low risk to customer data privacy. The tape in question was one of a number produced on a daily basis containing a snapshot of certain data held on some of the company's systems at a particular moment in time. Some back-up tapes don't contain any customer data at all," O2's statement read.
"The data on the tape is in a format that is not accessible to someone trying to access it and requires specialist technology to extract any readable information from it."
The company added that it was reviewing its tape-handling processes.