Virus writers are cashing in from turning other people's PCs into zombies sending out spam, MessageLabs has warned.

A report by the email security services company covering January to June this year shows a dramatic increase in virus writers teaming up with spammers to produce a more sophisticated breed of email security threat.

Since January, many viruses intercepted by MessageLabs - such as Mydoom and Bagle - have been found to have the potential for spam distribution.

Nearly two thirds (63 per cent) of all emails the company scanned were identified as spam - and one in every 12 of these contained a virus.

This is up from one in 208 in 2003 (0.5 per cent) and one in 392 in 2002 (0.3 per cent).

The company has also warned that spammers have started using spyware as a way of targeting more victims.

Paul Wood, MessageLabs chief information security analyst, said in a statement: "The boundaries between viruses and spam have been eroded, and commercial gain would seem to be the driving force.

"There is little or no profit to be gained from simply distributing viruses, but when you consider the income that can be earned from spam you have an altogether more attractive proposition."

Virus writers that used to be content with causing chaos and enjoying their 15 minutes of fame have realised spam's potential earning power, he added.

"They are using their virus-writing capabilities to hijack computers and create networks of zombie machines that send millions of spam emails.

"The preferred method of spamming is now via these botnets and the industry will pay big bucks to hire them out, so it has become cool to spam," he said.

The report also said the phishing phenomenon is also continuing to be a menace to internet users.