In fact, of all phishing messages currently analyzed, some 64 percent target Barclays' clients. Given the number of variants detected, estimates put the number of these e-mails in circulation at several million.

Jeremy Matthews, MD of Panda Software SA, explains: 'We believe this could be a coordinated attack, initiated in several places at the same time, in order to spread rapidly and gather a considerable amount of confidential bank details in record time.

'This is a very sophisticated attack in comparison with those that we usually see. The use of several domains to host spoofed Web pages makes it more difficult to disable them. The e-mails are also far more authentic looking than the usual, often error-strewn messages.'