Another variant of the SoBig virus is likely to appear some time after 10 September, antivirus company Sophos has warned.

SoBig F, which caused chaos globally last week, is the sixth version of the worm, although only that variant and SoBig A are still infecting PCs.

While the latest SoBig worm will automatically stop infecting machines on 10 September a new variant, which may also attempt to get infected machines to download code, is probably on the way after SoBig F self-destructs, predicted a Sophos spokeswoman.

"We are going to see a new version of the worm soon after," she said.

SoBig F contained instructions to try and download software from 20 sites between 20:00 and 23:00 GMT last Friday, and again on Sunday.

Although the download windows on Friday and Sunday passed without incident, two more download windows will open this coming Friday and Sunday.

Sophos said that the worm was still clogging networks with the volume of traffic it creates. The antivirus company advised companies to install software that blocks executable code at the email gateway.