The Ministry of Defence computer hard drive lost by IT services company EDS may have contained details of 1.7 million people, the government has admitted.
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The loss came to light earlier this month, but only now has the possible total number of compromised personal details been disclosed.
The MoD and EDS issued a statement saying they have failed to find the hard drive. They admit banking details of some of those affected were on the disc, and they are working with payments clearing organisation Apacs to help protect people's accounts from potential fraud.
The unencrypted drive went missing from EDS premises in Hampshire. The drive's loss was revealed during an ongoing audit of data security ordered by the Cabinet Office.
The MoD has lost hundreds of laptops over the past couple of years, which has resulted in a number of data security and national security scares.
Defence minister Bob Ainsworth admitted to MPs that in the "worst case scenario" the missing disc may have contained the personal details of 1.7 million people, mainly made up of potential recruits to the forces, along with serving members.