The comments from Major General Jonathan Shaw come in the same week that the website of the UK's (SOCA) was downed by a suspected DDoS (distributed denial of service) attack.

"The number of serious incidents is quite small, but it is there," Shaw told the Guardian.

"Those are the ones we know about. The likelihood is there are problems in there we don't know about."

Shaw declined to provide any more details about the nature of the serious breaches, or about who was responsible for them.

The MoD's cyber security chief, a veteran of the Falklands and Iraq wars, is due to retire soon. In the interview with the Guardian, he questioned the ability of senior military personnel to lead the fight against cyber attacks.