The “Troj/BankAsh-A virus” is the latest in “pharming” attack which divert people visiting legitimate bank websites to fake domain addresses owned by criminals.

Barclays, HSBC, Lloyds TSB and NatWest all feature in the attack list of the virus which is delivered to victims via attachments in spam e-mails. Unlike phishing, which relies on the user clicking on a link to a bogus website, the attack is triggered by the virus itself. This lies inside victims’ computers before automatically redirecting users to a fake website when they try to visit their internet bank. Once password details have been unwittingly revealed, the victims’ accounts are emptied.

Lycos.co.uk, the internet service provider, which screens its 25 million e-mail subscribers for viruses, has tracked and stopped 39,789 incidents of the virus since it first appeared on the internet about a month ago.

The perpetrators of the Troj/BankAsh-A virus scam, who are believed to be from Britain because British banks are the primary focus of the attacks, have not been tracked down.

Wessel van Rensburg, the head of e-mail at Lycos UK, said: “The use of this software is far more insidious than recent phishing attacks because it can be ‘seeded’ out to users’ PCs through viruses, worms or e-mail attachments without internet browsers knowing that it is lurking on their machine.”

The “weak link” of the virus was that it was easily detected by anti-virus filters, he added.

Security experts said that the virus would cease to be a threat if internet banks had to prove their identity to users logging on to their accounts, instead of relying on customers to key in password details.

Paul Docherty, a technical director of Portcullis Security Systems, a security consultant, said: “Unless you are technically savvy, you have to take it on trust that the website you are connected to is who you believe it to be. It would be quite straightforward for the banks to put verification systems in place which would enable users to know the website is for real.

“But the logistics of rolling this technology out to millions of internet bank customers are holding them back and there would also be a cost.”

Sandra Quinn, a spokeswoman for the Association for Payment Clearing Services, which represents the banks on payment issues, said that no banks had yet reported losses as a result of the virus.

She said: “We are looking at a range of security options for the future, but at the moment the systems are what customers want and they give them the security that they need.”

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