E-mail security vendor MessageLabs of New York City said it blocked more than 2.7 million e-mail messages with the new Sober variant since around 7 p.m. GMT on Monday in what it called a "major offensive."

Other E-mail security vendors have also raised the alert for the new Sober worm variant reports

Responding to the outbreak the FBI said: "These e-mails did not come from the FBI."

It added: "Recipients of this or similar solicitations should know that the FBI does not engage in the practice of sending unsolicited e-mails to the public in this manner."

The virus also comes in varieties that purport to hold a video of Paris Hilton, fake password change notices and e-mail error messages. It can only infect those using Windows PCs reports BBC

The first Sober virus was found in October 2005 and there have been 25 variants released since then. This latest variant checks to see if a machine has been infected by earlier versions and tries to shut them down so it can do its work.

Across the Atlantic Ocean, Austria's equivalent to the FBI is investigating a flurry of similar bogus e-mails sent in its name to people in Austria, Germany and Switzerland, the Associated Press reported.

"This particular virus is a mass-mailer worm and is the largest one we have seen this year," said Alfred A. Huger, senior director of engineering at Symantec Corp., which sells Norton AntiVirus software. "It's as bad as it gets. With this particular type of virus on your system, there is a high probability that your personal information will be stolen."

Anti-virus vendors advised customers to update their anti-virus signatures and to be wary of scam e-mail messages