Computer systems at Westminster have been infected by the Downadup virus.
The outbreak first came to light after an email by the parliamentary director of ICT was leaked online on Thursday night.
Engineers at parliament are working to "act swiftly to clean the computers that are infected", according to the email, the text of which was forwarded to ZDNet UK's sister site, silicon.com, by a parliamentary spokesman.
In the wake of the outbreak, computer users were warned by the parliamentary director of ICT to remove all unauthorised computers from the network "immediately", while politicians and civil servants have found their computer access slowed and have been locked out of some accounts as the result of the infection.
"We are scanning the network and if we identify any equipment which we believe is infected with the virus then we will contact you to ensure that the device is either removed from the network or cleaned and loaded with the correct software to prevent this infection reoccurring," the email said.
Staff have also been asked not to use any USB memory sticks or other portable storage devices for fear of spreading the virus.
Guy Bunker, chief scientist at security vendor Symantec, said: "It is really persistent and will transmit itself by any means necessary, it will find a way in."
Although the parliamentary spokesman refused to confirm which variant of virus had infected the systems, Microsoft last year issued a patch fixing the vulnerability exploited by Downadup.
Greg Day, security analyst at security vendor McAfee, said parliament needs to re-evaluate how it could best protect its network.
According to Day, parliament should be "making sure that security patch updates are always enabled, that early warning systems are in place and they are making use of signature-based techniques".
The Downadup virus, also known as 'Conficker', was responsible for infecting computers at five hospitals in Sheffield in January.
Talkback
Protecting a large network, such as this, is almost impossible when using windows based computers.
Ator said it is impossible to protect Parliament using Windows based computers - then what would Ator recommend? And when he makes his recommendation, he should add who he works for and why he indicates Windows wouldn't be the proper way to secure an infrastructure.
I said, "almost impossible." My wife works for an international company that uses windows entirely. they have been hit by users on the internet, using IE.
They have been hit by e-mail trojans and viruses'. I work part time for a large international company that uses UNIX servers, and windows internally. They have been hit by users on the internet, using IE, but it has not been as bad as it could be if they were using windows servers. No OS is 100% secure, but it is a proven fact that windows computers are easier to hack than any other OS, whether you like it or not, facts speak the truth.
This incident highlights the added value of Application Control in automatically affording protection by preventing any untrusted software – that is not explicitly permitted by policy that’s been downloaded via the Internet, transferred via a USB stick or installed from a CD/DVD – from having the ability to execute on a users’ PC.