The scale of this attack has seen the number of fraudulent e-mails detected by PandaLabs increase by 30% in just a few hours.

PandaLabs says of all phishing messages currently circulating, some 64% are targetting Barclays' customers. Given the number of variants detected, estimates put the number of these e-mails in circulation at several million.

The spam e-mails are designed to appear as if they have been sent from Barclays' customer services.

The message text imitates Barclays' corporate image and says the bank its upgrading software and asks recipients to click on a link in order to confirm their bank details. Users that click on the link access a form, similar to those used by the bank, requesting their account number, credit card number or PIN.

There are 61 different variants of the message, using a wide range of message subjects and sender addresses. This tactic is used deliberately in order to bypass antispam systems, says Panda.

Luis Corrons, director of Pandalabs, says this is a sophisticated attack compared with those normally seen: "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."

Corrons says not all of the e-mail messages point to the same Internet address as the criminals have prepared at least five false domains.

Panda says it is contacting the technicians in charge of the sites that have been located - all of which are in Korea - in order to shut them.