Cyber criminals have managed to hack into some of the Ministry of Defence's top secret computer systems, the military's head of cyber-security has admitted.
Major General Jonathan Shaw said the number of serious incidents was "quite small" but conceded it was likely that some attacks had gone undetected.
In an interview with The Guardian, Maj Gen Shaw said the level of cyber attacks were "still on an upward curve", meaning increasing the security of the military's computer networks was now a top priority.
"The number of serious incidents is quite small, but it is there," he said.
"And those are the ones we know about. The likelihood is there are problems in there we don't know about."
Maj Gen Shaw said that next year's MoD budget was expected to include new money to improve cyber-defence despite widespread cutbacks.
"I think it was a surprise to people this year quite how vulnerable we are, which is why the measures have survived so long in the (budget) because people have become aware of the vulnerabilities and are taking them seriously," he said.
Maj Gen Shaw, who has spent the last year reviewing the MoD's approach to cyber-security, also said the MoD had to embrace unconventional ideas and develop an array of its own cyber-weapons if it wanted to catch up with and then stay ahead of its rivals.
"It is quite right to say that pure defence, building firewalls, will not keep the enemy out. They might be inside already...there is no such thing as total security.
"One needs to engage in internal defence and be quite aggressive about it. And if you are going to manoeuvre in cyberspace, that is something that obviously involves action across the spectrum."
Earlier this week the The Serious Organised Crime Agency was forced to take its website offline on after being targeted by cyber criminals, while the The Home Office website fell victims to a denial of service attack last month.
Cabinet Office minister Francis Maude yesterday revealed that UK Government computer networks are "regularly targeted" by foreign intelligence agencies and groups working on their behalf.Suggest a correction