The Department of Work and Pensions is set to investigate the loss of two discs that are believed to hold data on thousands of benefit claimants.
Newspapers have reported that the discs – which were not encrypted – have been missing for a year because a contractor forgot to return them.
A DWP spokesperson would not confirm or deny whether a contractor was involved or what was held on the discs, which are expected to be returned to the DWP imminently.
But the department said it would investigate the loss, which came to light in the wake of HM Revenue and Customs’ loss of 25 million people's data on two CDS.
"We take the security of customers' data extremely seriously. Although there is no indication that any customers' data was compromised by this incident we are investigating and will ensure safe return of the information," the DWP spokesperson said.