The Conficker C internet worm could strike at infected computers around the world on April 1, a security expert warned on Monday.

Conficker C is a sophisticated piece of malicious computer software, or malware, that installs itself on a PC hard drive via specially written web pages. It then conceals itself on a computer.

Graham Cluley, of the security specialist Sophos, confirmed that Conficker C is programmed “to hunt for new instructions on April 1”. However, he added, “This does not mean that anything is going to happen, or that the worm is actually going to do anything. Simply, it is scheduled to hunt a wider range of websites for instructions on that date.”

One strange thing about Conficker C is that no one yet has any idea what it is programmed to do. In February, Mr Cluley told The Times: “It’s as if someone is assembling an army of computers around the world, but hasn’t yet decided where to point them.”

A worst-case scenario for April 1 would be for all the world's millions of infected computers to receive simultaneous instructions to attack, or to flood the internet with spam e-mail.

Ed Gibson, Microsoft’s chief security adviser for the UK, was reluctant to make predictions about Conficker’s behaviour. “April 1 is a classic date for anything like this to go off,” he said. “But I really would hate to say that April 1 is going to be unlike any other day.”

Mr Gibson reiterated Microsoft’s advice to consumers, that the best way to protect themselves against Conficker and any other worm or virus is to keep their system up to date. Microsoft maintains a special site that will remove infections from a suspect PC. Millions of PCs around the world are believed to be infected.

Mr Cluley of Sophos additionally suggests that internet users stick to internet pages they know and trust. “We suspect that the people seeding these viruses are monitoring search trends on Google, then hastily putting together special sites that contain clippings from legitimate sources, but also install malicious code.” He also advised internet users to look out for fake PC security scanning websites.

For full details on the Conficker worm, see Microsoft's dedicated page.

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