Botnet-controlled PCs are responsible for 83 per cent of all spam messages, according to the latest figures from MessageLabs.

The security firm said in a recent report that, despite efforts to shut down service providers notorious for hosting shady operations, spam vendors are continuing to send out huge loads of junk mail via botnet-infected systems.

Researchers estimate that spam levels in June reached 90 per cent of the total global email load, holding steady from May. It is estimated that 83.2 per cent of those spam messages were coming from botnets.

The biggest offender by far was the huge Cutwail botnet, which accounted for some 45 per cent of all botnet spam on the web. Despite the shutdown of one of the botnet's main hosts, Cutwail was able to assemble a network of as many as 2.1 million machines and send out an estimated 35 per cent of all spam emails worldwide.

According to MessageLabs, the majority of Cutwail's infected machines reside in Brazil, the US and Korea, all popular countries for malware and botnet infections.

Second to Cutwail in spam volume was the Mega-D Trojan, which logged 9.5 per cent of the botnet spam load. In the third spot was the Grum worm with six per cent, followed by the Rustock malware with four per cent.

A number of smaller, unclassified botnets accounted for an additional 10.5 per cent of botnet spam attacks.

Researchers noted that webmail systems have also become a target for spammers.

"Much of the remainder is sent from compromised mail servers and webmail accounts," read the report.

"Some of the smaller botnets can also control the sending of spam through webmail accounts in such a way as to make it appear as though there is a real person behind the use of each webmail account."