Antivirus companies warned Tuesday that a new version of the Sobig virus is rapidly spreading on the Internet, the latest in a string of Sobig computer worms to be released.

The new worm, W32.Sobig.F, first appeared on Tuesday, prompting antivirus software companies to release updated virus identity files to detect and stop the new threat.

F-Secure of Helsinki rated Sobig.F a "Level 2 Alert," indicating a large number of infections. Sophos said that it had received "many reports" of the latest Sobig worm from customers.

The first Sobig worm appeared in January, infecting machines running Microsoft's Windows operating system.

Like that worm, Sobig.F spreads through infected e-mail message attachments and unprotected shared folders on computer networks, modifying a computer's operating system so that the Sobig.F worm code is run whenever Windows is started, antivirus companies said.

When opened, the worm places a copy of itself into the Windows folder on the infected machine, creates a process to run the worm program and modifies the Windows registry so that the worm program will be launched whenever Windows is started.

Sobig.F, like its predecessors, comes with its own SMTP (Simple Mail Transfer Protocol) engine which it uses to mail copies of itself to e-mail addresses it skims from file and e-mail address books on a victim's computer, Sophos said.

The worm arrives in e-mail messages with nondescript subjects such as "Re: Thank you!" "Your details" and "Re: wicked screensaver." The worm code is stored in attached executable files with names such as "your_document.pif," "details.pif" and "movie0045.pif," according to F-Secure.

However, unlike earlier strains of Sobig, the F-strain is more savvy in its efforts to trick users into opening the infected file that launches the worm.

All versions of the original Sobig worm were sent from the same e-mail address, big@boss.com, and a later variant posed as an e-mail message from Microsoft Chairman and Chief Software Architect Bill Gates. In contrast, Sobig.F inserts e-mail addresses stolen from the victim's computer into the "From:" field, creating the impression that the e-mail was sent from a trusted source, F-Secure said.

Like earlier Sobig variants, Sobig.F comes with an expiration date. The worm will stop spreading on Sept. 10. Copies of Sobig.F that are launched after that date will shut down immediately, F-Secure said.

In the past, new Sobig strains have appeared soon after previous strains expired.

Antivirus companies recommend that customers update their antivirus software and have posted instructions and free tools for disinfecting machines infected by Sobig.