Up to half of all spam filtered by security firm Marshal is caused by the Srizbi botnet, according to latest figures from the company.

Marshal's statistics show that the Srizbi botnet has compromised almost a third of a million PCs worldwide, dispatching an estimated 60 billion spam emails per day using subject lines on everything from 'watches' and 'pens' to 'male enlargement pills'.

"Srizbi is the single greatest spam threat we have ever seen," said Bradley Anstis, vice president of products at Marshal.

"At its peak, the highly publicised Storm botnet only accounted for 20 per cent of spam. Srizbi now produces more spam than all the other botnets combined. "

Srizbi sprang to fame in March when a Marshall report identified it as one of a small group of 'super-bots' starting to monopolise global internet traffic.

Anstis pointed to the industry-wide success in repressing the Storm botnet, which appeared early last year, as a glimmer of hope in dealing with the new threat posed by Srizbi.

"Microsoft recently announced its success in combating the Storm botnet with its Malicious Software Removal Tool," he said.

"The challenge now is for the security industry to collectively turn its sights on Srizbi and the other major botnets. We look forward to seeing Microsoft target Srizbi in the near future."