A computer virus spread via e-mail has been described by security experts as the "largest virus outbreak in months".
The malicious worm, called Mydoom or Novarg, has clogged networks and may allow unauthorised access to computers.
It arrives as an e-mail attachment in a text file which sends itself out to other e-mail addresses if opened.
Security experts MessageLabs said, at its peak, one in 12 e-mails carried the worm. It has now stopped more than 1.5 million copies of it.
It said this latest rapidly-spreading worm is bigger and faster than Sobig.F, the virus which crippled inboxes and networks last August, and it shows little sign of slowing.
"Sobig, at its peak, infected one in every 17 e-mails, causing many internet relays to become severely clogged," Mark Sunner, chief technology officer at MessageLabs told BBC News Online.
"Mydoom has now surpassed this, infecting an incredible one in every 12 e-mails, and so the impact of this latest virus may be very serious for affected e-mail users." "The sender is therefore falsified in the Mydoom virus, so it is impossible for the recipient to actually tell where the e-mail has really come from."
Thousands of e-mails triggered by the worm, which only affects computers using Microsoft Windows, were bombarding networks within hours of its discovery on Monday.
The worm is similar to 2003's Bugbear and Sobig in the way it spreads, Symantec's Kevin Hogan explained to BBC News Online.
"It is a very simple example. It simply relies on a human to double click on an attachment to run it."