The Ministry of Defence (MoD) said that it is investigating the breach, which is believed to stem from the loss of three portable hard drives from an RAF base at Innsworth in Gloucestershire.
The MoD said that two of the three drives contained RAF personnel records, while the third did not hold any sensitive information. The drives are reportedly carrying details on some 50,000 people.
The MoD does not believe that the stolen data is currently being used for any fraudulent purposes.
"An urgent assessment is underway into the detailed nature of the data that was on the drives," said the MoD.
"The theft of these hard drives from a secure location, where they were subject to physical protection standards consistent with the Data Handling Review, is being treated with great seriousness."
The incident is the latest in a rash of data losses by law enforcement and government groups. Earlier this year, Hertfordshire police lost some 330,000 criminal records contained on a USB stick.
Among the largest of the breaches came last year, when the HMRC lost two discs holding the records of some 25 million people.