It's one of the nastiest I've seen

Martin Eddolls, Internet Marketing Management

The virus, believed to have originated in the Philippines, caused UK giants such as BT, Vodafone, Barclays and Scottish Power to block incoming and outgoing e-mails.

It appeared in the form of an 'Iloveyou' e-mail with an attachment that people were encouraged to open.

Martin Eddolls, of Internet Marketing Management, described it as "one of the nastiest I've seen".

Those most affected financially are likely be companies which rely on e-mails for business transactions.

'Losing money'

Graham Cluley, of the anti-virus computer company Sophos, said: "Companies blitzed by this thing are switching off their e-mail systems.

"As soon as they do that, they are losing money because they are effectively turning off the cash flow, being unable to talk to customers by e-mail.

"It's like having a power cut. The costs, depending on the business, will be extremely large."

The spread of the Love Bug, which when opened replicates itself via a computer's e-mail address books, was prolific.

Virus technologists at MessageLabs said it caught 6,000 copies of the bug on Thursday. This compared to only 1,600 copies of last year's infamous Melissa virus caught over 12 months.

A BT spokesman said the company's 80,000 e-mail users had been "marginally" affected by the bug but staff were trained to spot computer viruses.

Back to faxing

A Barclays spokeswoman said some servers had to be shut down, while AT&T was putting remedial software into action to purge the bug overnight.

For many companies, it meant a return to the more traditional forms of communication, such as phone or fax.

A spokesman at Ford UK, which shut down its internal communication system for several hours, said staff were forced to get on with some more paper-focused tasks such as filing.

Even the House of Commons was cut off from electronic communication with the outside world, as the network was shut down to prevent the bug spreading.

It is believed the virus is programmed to delete some computer files, including MP3 music files and images, as well as raiding email addresses to multiply itself and send itself and other e-mails onwards.

Once opened it forwards itself using the personal and business address books of e-mails so the spread is rapid.

Internet music firm peoplesound.com was among those hit. It was forced to wipe its entire files, and painstakingly replace them from its back-up system, after technical staff the scale of the problem.

No hiding place

"It's a particularly malicious virus," said technical director Andrew Fisher.

For many companies around the globe, there was no hiding place.

Sally Watson, an editor of Silicon.com, an online news network aimed at IT professionals, said: "Usually viruses hit the United States first and we get 12 hours' warning. But this time Europe and Asia were the first to get infected.

"The Melissa virus cost many millions of pounds here and we think this one is going to be bigger, because there was so little warning."

Alan Stevens, head of digital services at the Consumers' Association, said: "This has been done as a joke, but the worrying aspect is that cyber terrorism aimed at a company or a country has become relatively easy to do. We haven't seen the last of it."