The Sober.P worm is circulating the Internet in greater quantities than ever, according to antivirus company Sophos.

Sophos says the mass-mailing worm accounted for 5.4 percent of all e-mail the company saw over the weekend and 84 percent of virus activity. That represents an increase compared with Friday, when Sophos said the worm accounted for 4.65 percent of all e-mail and 77 percent of virus activity.

"The strange thing is that we're actually seeing more reports than ever," said Graham Cluley, senior technology consultant for Sophos. "It's increased and it's even worse than last week. We don't know how many people are infected, but those infected are just spewing these e-mails out."

Cluley said that the second-most-prevalent virus, Netsky.P, accounted for 0.3 percent of all e-mail viruses, and Zafi.D, the third-most-common worm, was just 0.082 percent. "Those have been big viruses, but have been dwarfed by the Sober worm," he said.

Last week, Sophos said that the worm turned off Symantec's antivirus protection and Microsoft's Windows XP firewall on infected machines.

Sober.P -- which security companies have variously tagged as Sober.N, Sober.O and Sober.S -- travels as an attachment in e-mails written in English and German. One of the most widely reported e-mails contains an alluring message stating that the recipient has won free tickets to the 2006 World Cup in Germany, but many other types have also been spotted. Once opened, the virus sends itself to e-mail addresses harvested from the newly infected machine.

ZDNet UK's Dan Ilett reported from London. For more coverage from ZDNet UK, click here.