The number of compromised computers jumped to more than two million last year, with 25 per cent of all malicious attacks linked to botnet activity, new figures show.

This has led to an increase in the number of organisations that experienced large scale spam attacks, according to security experts at Network Box. Last year spam levels rose by 200 per cent, with a leap of more than 50 per cent in the third quarter alone.

Moreover, hackers are using increasingly complicated software on botnet controlled computers, adding random words and images to confuse spam filtering devices.

"We have seen bugs in spam code that stops the payload being loaded successfully," said Simon Heron, technical director at Network Box. "However, trojan software is now sophisticated enough to look for updates to remove these bugs. These campaigns will become much more successful in 2007."