Computer virus expert Jason Holloway: The bug can be eradictated

The virus, called either Zipped Files or ExploreZip, is sent via e-mails and attacks computer files.

It is spreading rapidly as, once opened, it automatically sends itself as a reply to any e-mails in the in-box of the infected computer.

The virus has already hit a major investment bank in the City of London, and is thought to have affected BT and BSkyB among others.

Delete instantly

Action was taken on Friday to intercept the virus after it was found at the BBC.

The virus arrives in a personalised message with the text: "Hi (recipient's name). I have received your e-mail and I shall send you a reply ASAP. Till then, take a look at the attached zipped docs. Sincerely, (sender's name)."

If the file is opened, the virus will erase documents on the computer's hard drive written with standard Microsoft software such as Word, PowerPoint and Excel.

A statement from Microsoft, warning of the worm, said: "Customers receiving this e-mail message should delete it immediately without opening the attachment and then empty the deleted items folder."

Credible-looking

Microsoft is believed to have cut off its corporate e-mail connection with the Internet for two hours on Thursday to prevent it from becoming infected.

In March, computer networks were hit by the Melissa virus, which oveloaded networks by sending hundreds of e-mails.

The ExploreZip worm has been reported in more than a dozen countries, including the US, Germany, Norway, Israel and the Czech Republic.

Mikko Hypponen of anti-virus software company Data Fellows, said: "The key issue here is that messages sent by ZippedFiles are very credible, normal-looking replies to messages you have sent earlier.

"You're quite likely to trust these messages and open the attachment." Patches can be downloaded from the Web from various computer security firms to identify and stop the worm.