One of the biggest global operations to disable a number of botnets believed to be responsible for the theft of millions of pounds is underway Microsoft has said.

The action taken by Microsoft has been labelled as its “most complex effort to disrupt botnets to date” Microsoft is working in collaboration with financial services to disable botnets powered by Zeus malware.

Senior attorney for Microsoft’s Digital Crimes Unit Richard Bosgrove gave details of the operation in his latest blog post. “Cybercriminals have built hundreds of botnets using variants of Zeus malware,” he wrote.

“For this action – codenamed Operation b71 – we focused on botnets using Zeus, SpyEye and Ice-IX variants of the Zeus family of malware, known to cause the most public harm and which experts believe are responsible for nearly half a billion dollars in damages.”

Microsoft estimates that more than 13 million computers have been infected with Zeus malware worldwide.

Botnets like Zeus are created by the spread of malicious software, gaining access often through infected emails or vulnerabilities within web browsers.

Each “bot” is a hijacked computer which can be used by hackers for any number of illegal activities.

Users can protect themselves by regularly updating security software and by being wary of attachments form senders they do not know or trust.