The company targeted by the MyDoom worm has placed a $250,000 bounty on its authors head.

SCO offered the reward as the scale of the attack against it became apparent throughout Wednesday.

Experts fear MyDoom has already surpassed the rate of infection managed by last year's Sobig virus.

Mark Sunner, chief technology officer at email security company MessageLabs, said MyDoom accounted for one in 12 emails in circulation on Wednesday, compared with SoBig's one in 17 at its peak.

"Mydoom has just surpassed Sobig.F as the fastest spreading virus ever," said Mr Sunner. "With a text file icon instead of graphics that lead people to believe it is innocuous, this virus appears to have hit a sweet spot in execution and propagation.

The virus is programmed to start bombarding SCO's network with data from infected machines (known as 'zombies') on 1 February. This is called a distributed denial of service attack, because the computers from all over the world are involved. The weight of traffic sent to SCO could force its computers to shut down, causing inconvenience for the company.

SCO is involved in legal arguments with companies that use the open source operating system Linux, and insists that it owns some of the code used in the software. The move has caused great controversy in the open source community.

MyDoom, also known as Mimail-R, spreads via email, using a variety of technical-sounding subject lines and attachment names.

If the attached file is launched and the worm activated, it scours hard disks for more email addresses to send itself to. MyDoom also opens a backdoor onto infected computers which allows hackers to gain access.

Unlike earlier worms, MyDoom does not attempt to entice users into opening attachments that promise pornography, gossip or prizes. Instead, it appears to be a 'test' email, and may bear the header "Status" in the subject line.

To be safe, do not open attachments unless you know exactly who they are from and make sure your anti-virus software is up to date.