"We are aware of it and we are actively investigating," an FBI spokesman said last night.

The new bug was first spotted on Monday and had infected hundreds of thousands of computers around the world by yesterday morning, said Mikael Albrecht of the Finnish anti-virus firm F-Secure.

It uses the e-mail addresses stored on infected computers to replicate itself and spread further, Mr Albrecht said.

The virus arrives on e-mails with messages such as: “Mail transaction failed. Partial message is available.” If the user opens the accompanying file, the virus W.32.Novarg.A@mm, is activated and sent to everyone in the address book.

MessageLabs, a computer security company, said that as many as one in 12 emails sent worldwide yesterday were infected.

Its main purpose, Mr Albrecht added, was to attack and overload the web site of SCO, Santa Cruz Operation, one of the world's biggest sellers of the Unix operating system, a competitor to Microsoft's Windows.

The bug's secondary function was to provide its author with a "back door" to the infected computers to control them remotely, possibly to co-ordinate an attack, analysts said.

The FBI last year arrested at least two people believed to be responsible for the "Blaster" virus that created havoc on the Internet and are currently hunting for authors of the SoBig e-mail worm, which paralysed some portions of the internet last year.

Regarding the MyDoom virus, the spokesman said, "We have not done a full assessment, but it's serious enough to warrant the FBI to look into this."

Some analysts are comparing the new virus to SoBig and have said that it may be the fastest-spreading worm ever.

"Initial indications are that MyDoom is propagating as fast as SoBig.F," said Scott Chasin, chief technology officer at US-based computer security firm MX Logic.

The company has seen a peak of 1,200 infected e-mails per second, making it a "critical threat".

Some analysts said that people were opening the attachments and spreading the virus because the e-mails appeared innocuous, sometimes referring to failed mail deliveries.

The New York-based computer security company MessageLabs said that it had intercepted 1.8 million copies of the virus.

SCO Group said that it was offering a $250,000 (£136,000) reward for information leading to the arrest and prosecution of the virus creators.