The Ministry of Defence was today investigating its worst information security breach after a portable hard drive with details of about 100,000 servicemen and women and 800,000 applicants to join the Armed Services was found to be missing on Wednesday.

Sensitive details of the family members of personnel were also among the data stored, including bank details and passport numbers.

The portable hard drive — which is believed not to have been encrypted — was used by EDS, the MoD’s main IT contractor, to test computer equipment. It could have been missing for several days.

“The matter is being investigated by MoD police,” an MoD spokeswoman said last night. “We were informed by EDS that it was unable to account for a portable hard drive used in connection with the administration of Armed Forces personnel data.

"This came to light during a priority audit EDS is conducting to comply with the Cabinet Office data handling review.”

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, plus their next-of-kin details.

Officials are “not ruling out” the risk that bank account details of personnel were held on the drive. It also has data on 600,000 potential services applicants and the names of their referees.

The department said it learnt 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.

The Liberal Democrats described the loss as a “disturbing breach of security” and called for an urgent inquiry into how it had occurred.

Dr Liam Fox, Shadow Secretary of State for Defence, said: “The MoD has a very poor record on keeping sensitive and classified data secure. This latest scandal is yet another example of poor operational security within the department."

Nick Harvey, the Liberal Democrat shadow defence secretary, 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.”

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