Spam levels appear to be on the rise again after a brief improvement following the shutdown of unethical network McColo in November 2008.

An official Google Enterprise blog post by Amanda Kleha, from Google's Message Security Team, shows that spam, while still down on 2008 levels overall, is up by 156 per cent since November.

These statistics follow the 70 per cent drop in spam experienced after McColo was taken offline, according to Google figures.

Mr Kleha also indicated that the rate of growth is likely to accelerate in the future, stating: "As spammers recover, the increased rate of spam growth will likely have total spam volumes back to pre-McColo levels within a few months."

These findings highlight the importance of anti-virus software for consumers over the coming year. Ms Kleha said: "Our statistics show that the average unprotected user would have received 45,000 spam messages in 2008 (up from 36,000 in 2007). "All indicators suggest this trend will continue as virus, malware and link-based attacks become more frequent and more ingenious."

The post from Google follows a series of reports released at the end of last year, which suggested that spam attacks in 2009 would continue to increase in complexity and sophistication. One study from internet security firm Barracuda Networks predicted that spam could exceed 95 per cent of the global volume of emails in 2009.

Google repeated this warning in its post, with Ms Kleha warning: "Despite eliminating a major source, spam keeps coming back. Spammers are re-investing with increased speed to evolve their systems into decentralised, harder-to-detect ecosystems."