The Ministry of Defence has admitted that staff leaked secret information 16 times on social networking sites such as Facebook and Twitter over an 18-month period.
Lewis said the Ministry of Defence had disciplined 10 personnel, although was unable to specify individual cases.
Ministry of Defence staff aren't banned from using social networks, but Lewis pointed out that the department's code tells employees: "Remember you are a member of HM Forces/MOD civil servant. Observe the same high standard of conduct and behaviour online as would be expected of you in your professional or personal life."
However, F-Secure said the Ministry of Defence should do more to ensure the guidelines are adhered to.
"It's worrying that employees in sensitive positions have been sharing confidential information via Twitter and other means," said F-Secure's security expert Mikko Hypponen
"They might think they are confiding in friends or family when they go on Facebook. However, the recent changes in Facebook's privacy settings might make them disclose information to the world. This is a potential security risk."