Britain's armed forces have no proven back-up if a cyber attack downs vital information and communications systems, the government has been warned.
A Commons defence select committee report said that a sustained cyber attack from enemies could leave the military "fatally compromised" and demanded that the government act.
The coalition government has already pledged £650m to prevent and tackle cyber attacks and has also named cyber defence as a "tier one" strategic priority.
However, the select committee report says more needs to be done and plans need to be put in place quickly.
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The report said: "The evidence we received leaves us concerned that with the armed forces now so dependent on information and communications technology, should such systems suffer a sustained cyber-attack, their ability to operate could be fatally compromised.
"It is not enough for the armed forces to do their best to prevent an attack. The government should set out details of the contingency plans it has in place should such an attack occur. If it has none, it should say so – and urgently create some."
The report demanded "increased ministerial attention," called for offensive cyber weapons to be built and said the drawing-up of cyber-security rules of engagement were in urgent need.
Reacting to the report, shadow defence minister Jim Murphy added: "Policy progress is falling behind the pace of the threat our armed forces face. Vulnerabilities must be tackled urgently and ministers must respond in detail to the demands in this report."