Thieves have stolen files containing the personal records of thousands of serving and former RAF staff, the Ministry of Defence said.
The information was stored on computer hard drives at the Service Personnel and Veterans Agency at RAF Innsworth, Gloucester.
Police are investigating the incident which took place last Wednesday within a high-security area at the base. The MoD said it is treating the breach "extremely seriously".
The agency provides support services for some 900,000 serving and ex-service personnel.
It is as yet unclear what information was stored on the three disc drives taken and how many people's records are affected.
A spokesman for the MoD said: "We can confirm that an investigation is being conducted by the MoD Police, with the support of Gloucestershire Police into the apparent theft of three USB portable hard disc drives.
"We are taking this incident extremely seriously. In view of the ongoing nature of police inquiries, it would be inappropriate to comment further at this stage."
The data loss will come as the latest embarrassment for the Government, which has already suffered a series of lost or stolen data files. Earlier this week, a disc containing the names and addresses of almost 11,500 teachers went missing in the post.
Commenting on the latest data loss, Nick Harvey, Lib Dem Defence spokesman, said: "This is just the latest in a seemingly endless stream of stories involving personal information being lost or stolen.
"The first priority has to be ensuring that nobody's security has been put at risk, but we also need a serious look at the way such sensitive data is being handled. The current situation is unacceptable. How can the Government expect people to support ID cards when personal information is apparently so insecure?"
