A missing computer hard drive may have contained details of 1.7 million people who had enquired about joining the armed forces, the government has said.
The drive was reported missing last week by EDS, a firm contracted to the Ministry of Defence.
Armed Forces Minister Bob Ainsworth told MPs the information, which went missing at an EDS site in Hampshire, was "unlikely" to have been encrypted.
It is not known if the drive was lost or stolen. An investigation has begun.
In a written statement, Mr Ainsworth said the drive "may, in the worst case, contain details relating to 1.7 million individuals who have enquired about joining the armed forces".
While some who had made "casual enquiries" would only have given a name and contact details, "more extensive personal data" may have been held on those who had applied to join, he said.
"In some cases this will include personal information such as next of kin details, passport and National Insurance numbers, drivers' licence and bank details and National Health Service number," he said.
"EDS assesses that it is unlikely that the device was encrypted because it was stored within a secure site that exceeded the standards necessary for restricted information."