A security expert at Sophos has reminded former Microsoft chairman Bill Gates that his predictions about the end of spam have failed spectacularly.

Gates declared in 2004 at the World Economic Forum in Switzerland that spam would be "a thing of the past" within five years.

However, Graham Cluley, senior technology consultant at Sophos, wrote in a blog post that "with the prophecy's five-year anniversary approaching, spam continues to cause a headache for companies and home users".

Spam is in fact worse than ever, according to the security expert, and has become more sophisticated, more malicious and a greater threat to the security of nearly every PC user in the world.

"Virtually all spam, of course, is sent from innocent computers that have been compromised by hackers," Cluley wrote.

"Sophos Labs has been examining its spam traps around the world to get a picture of where these botnet computers, sending the unwanted adverts and malware, are based."

Where they come from is Gates's home turf, according to Sophos. The US sends or relays most spam, and Cluley said that, although the amount of spam sent via the country has fallen, it has not done so to the extent of other locations.

"Although the US spam contribution has significantly decreased since Bill Gates's proclamation - falling from almost half of all spam relayed at the end of 2004, to 21.3 per cent by the end of 2007, and now resting at 19.8 per cent - this shows that there is certainly no quick fix," he said.

However, Cluley maintained that end users could be doing more to protect themselves. "For heaven's sake, if you see someone ever buying goods advertised via spam give them a flick around the ear. They are only helping to perpetuate the problem," he said.