Ministry of Defence (MoD) computers have been hit by a severe virus that affects email and internet access.
The Royal Air Force and the Royal Navy are among the organisations whose machines have been affected.
"Obviously with a computer system of our size we are fighting off viruses daily, but not of this scale," the spokeswoman said. "I don't think we've ever had an instance like this before."
However, the Ministry revealed the virus has not jeopardised war-fighting systems and due to pre-existing security systems, no classified or personal data was compromised.
Just 27 percent of the Ministry's computers meet current data security standards for holding classified information and personal data, it said earlier this week. About 31 percent of systems meet some standards, while the rest are being evaluated.
Efforts to contain and clean up the virus have resulted in widespread shutdown of systems, but the ministry declined to say how many machines in total are affected. A solution to prevent re-infection of the PCs is being tested.
"The reason why so many people are without their computers is because we've turned them off rather than they've been wiped or destroyed by this virus," she said.
Some Navy systems are now up and running, but the Ministry did not have an estimate of how many of those systems remain down. It declined to say which warships have been affected, but news reports singled out the fleet flagship HMS Ark Royal, an aircraft carrier that went into service in July 1985.
Due to security reasons, the type of virus has not been publicly released, the spokeswoman said. However, the computer security community has been grappling lately with the Conficker worm, which targets a flaw in Windows Service Server, a component in Microsoft's Windows 2000, XP, Vista, Server 2003 and Server 2008 products.
Systems become infected when a hacker constructs a malicious Remote Procedure Call (RPC) to an unpatched server, which then allows arbitrary code to run on a machine.