Security experts today warned of a newly discovered variant of the Sober worm that is spreading using a relatively sophisticated social engineering attack.

The Sober N worm arrives as a ZIP file attachment in an email in either English or German. The email claims that the sender has been receiving your emails and has enclosed 10 in a file for your attention.

"The new Sober variant has been seeded last night," said Mikko Hypponen, director of antivirus research at F-Secure.

"Spreading speed is unknown at this time, but many previous Sober variants have been fairly big problems."

His company has raised the threat level of the worm to its second highest setting and is urging all users to update their antivirus software.

Once activated the worm harvests email addresses and sends itself onwards.

The first Sober worm was detected in 2003 and since then many variants have been spawned after easy to use virus creation kits for the virus started to appear on the internet.

Its variants went on to top the infection charts regularly in 2003 and 2004.