A computer hard drive with the private details of armed forces personnel is missing, the Ministry of Defence has confirmed.
The portable drive contains the names, addresses, passport numbers, dates of birth and driving licence details of around 100,000 serving personnel across the Army, Royal Navy and RAF, along with details of their next of kin.
It also contains data on 600,000 potential services applicants and the names of their referees.
The drive could not be accounted for during an audit conducted by MoD contractor EDS as part of the ongoing Cabinet Office review of data security being conducted by Sir Edmund Burton.
Officials said they were "not ruling out" the risk that bank account details of personnel were held on the drive, which belonged to its IT contractor EDS.
The department said it learned of the loss on Wednesday and MoD Police were investigating.
The missing drive is the latest information security breach to hit the MoD. In July it admitted 658 of its laptops had been stolen over the past four years and 26 portable memory sticks containing classified information had been either stolen or misplaced since January.
An MoD spokesman was unable to confirm reports that the missing data was not encrypted.
"On Wednesday 8 October we were informed by our contractor EDS that they were unable to account for a portable hard drive used in connection with the administration of armed forces personnel data," he said
"This came to light during a priority audit EDS are conducting to comply with the Cabinet Office data handling review. The MoD Police are investigating this matter with EDS."
The Liberal Democrats described the loss as a "disturbing breach of security" and called for an urgent inquiry into how it had occurred.
Shadow defence secretary Liam Fox said the loss highlighted the government's "reckless" approach to the security of military data. "The MoD has a very poor record on keeping sensitive and classified data secure," Fox said.
"This latest scandal is yet another example of poor operational security within the department."
Liberal Democrat defence spokesman Nick Harvey said: "This is extremely serious news and the immediate priority must be to ensure the security of all military personnel.
"However, once that has been established, there must be an urgent investigation into how such a disturbing breach of security could be allowed to happen."
News of the missing drive comes just days after it was announced that the IT contractor in question, EDS, would be shedding over 3,000 jobs in the UK after being taken over by Hewlett Packard in August.
