Experts are warning that the sharp rise in malware activity this week is being caused by hacking groups competing with each other to create large botnets of remote controlled PCs.

The worm attacks on Monday, Tuesday and Wednesday appear to come from three distinct hacking groups according to Mikko Hypponen, chief research officer at F-Secure.

"We seem to have a botwar on our hands," he said.

"There appear to be three different virus writing gangs turning out new worms at an alarming rate – it's as if they are competing to see who will build the biggest network of infected machines. The latest variants of Bozori even remove competing viruses like Zotob from the machines!"

What has made these worms different is the speed with which they were created. Microsoft released its patches on August 9 and the patches had been reverse engineered and exploit code was released by August 12. Since then three distinct worm types have exploited the vulnerability.

"Organised criminal gangs are behind attacks like these and their motive is to make money," said Graham Cluley, senior technology consultant for Sophos.

"Owning a large network of compromised computers is a valuable asset to these criminals and every business needs to take steps to ensure they are not the next victim on their list."

Experts are expecting a busy week ahead as the pool of unpatched PCs still remains a tempting target for hackers as many individuals and organisations have yet to finish the testing and installation of the new code.