O2 Ireland has warned that a lost backup disk may have contained customer data.

The mobile operator said yesterday that IT services partner IBM had, at some point over the summer, told it that a disk had been misplaced last September.

O2 does not know where the disk had been lost, or what data was kept on it. In a blog post on Tuesday, it said that there was a "low risk" that it might have contained customer data.

The data is not encrypted but O2 says that it is in a format that would require specialist technology to extract.

The company, which is owned by Spanish mobile operator Telefonica, said it has reported the incident to the Irish Data Protection Commissioner and informed its current and former customers of the data breach.

O2 conducted a "full review" of its backup tape handling processes and “stringent measures” have been taken to ensure it does not happen again, it says. It is not yet clear whether the disk was in IBM’s possession at the time of the loss.

The company was last involved in a data breach in January, when it emerged that the phone numbers of mobile web users on its network were passed to the websites they visited.

IBM has also been linked to lost backup tapes. In March, it emerged that a tape containing 800,000 child support records was lost in transit between an IBM storage facility in Colorado and an Iron Mountain site in California. And in 2011, IBM informed customer Health Net insurance that it could not find a disk containing personal data on 845,000 US citizens.