The Ministry of Defence (MoD) faces a "continuous battle" in cyber space with 1,000 attacks detected and blocked last year, according to Defence Secretary Liam Fox.
He revealed the extent of "potentially serious" attempts to infiltrate or disrupt the MoD's computer systems at a London Chamber of Commerce defence industry dinner.
It follows Chancellor George Osborne's disclosure that government computers are receiving more than 20,000 malicious attacks every month.
Fox added that "across the core defence networks there were an average of over a million security alerts every day", mainly spam emails, but including deliberate attempts to infiltrate MoD computers. He described it as the "war of the invisible enemy".
He said: "Our systems are targeted by criminals, foreign intelligence services and other malicious actors seeking to exploit our people, corrupt our systems and steal information.
"The risks to defence are real, and I take them very seriously," he added.
Fox did not reveal who was behind the attacks, but MoD sources indicated they involve hackers working for foreign governments, with China a leading suspect.
His disclosure came after Google revealed it had discovered an attempt to steal the email passwords of hundreds of account holders including US government officials, journalists and human rights activists.
Fox said that as well as government systems, major defence companies and other businesses have come under cyber attack, and he called for increased vigilance.
He said: "Our national intellectual property in defence and security industries is at risk from a systematic marauding.
"Not only could it severely affect the future success of British industry, our economic advantage, and the country's financial recovery – but it also directly impacts upon our national security today.
"This threat is growing in scale and sophistication – my department is a prime target."
The speech formed part of an explanation for a decision in last year's Strategic Defence and Security Review to increase spending on cyber warfare by £650m despite severe cuts in conventional forces.
A week ago Armed Forces Minister Nick Harvey disclosed that the military's work would include the development of offensive cyberwarfare capabilities.
The MoD has said it is looking to recruit hundreds of cyber experts and that the US has warned a cyber attack could be taken as an "act of war".