Some security analysts fear that the Conficker virus, which has infected at least 15 million computers across the globe, could have been programmed to trigger a series of further attacks today, perhaps including sending spam, crashing websites or even accessing personal files stored on computer hard drives, according to

Anti-virus experts have warned computer users to remain vigilant, but said they are not expecting “anything dramatic” to happen.

“There has been a lot of speculation about a mass activation of the Conficker worm, but as midnight struck across the globe, we have so far seen no evidence of any malicious activity on a mass scale,” said Greg Day, a security analyst for McAfee. “But this does not mean that users should have a false sense of security. It is important to deploy the latest security software and download current patches in order to protect your systems.”

Rob Cotton, chief executive of NCC Group, a leading IT security service, said that while it was possible the Conficker virus could be activated at some point to cause major distruption to computing networks, there was little evidence to suggest that activation would happen today.

“The likely situation at the moment is that the significance of today’s date is linked to a new version of the virus, Variant C, communicating back to the rest of the botnet, using a more sophisticated technique than we have previously seen,” he said. “With millions of computers already infected by the virus, and the number of people joining the internet rising daily, the potential of the virus to cause havoc is great. While today didn’t give us the dramatic showdown that some people anticipated, it should serve as a timely reminder for everyone to check and upgrade their security provisions.”

The Conficker worm, also known as Kido and Downadup, burrows its way deep in to the operating system of computers, providing hackers, spammers and cybercriminals with a ‘back door’ into people’s machines.

Computers at the House of Commons have already fallen victim to the worm, as have some computers in defence departments in the UK, France and Germany.

Anti-virus companies have released removal tools, while Microsoft has issued a patch to protect computers from the virus. The software giant has also offered a $250,000 (£172,000) reward for information that leads to the capture and conviction of the authors of Conficker.