A computer virus that has been spreading around the world for months is set to come alive tomorrow.

Experts have struggled to tackle it and they do not know who controls it or why it was created. But tomorrow the virus, dubbed Conficker, will “call home” to its creator to seek new instructions. No one knows what will happen next.

“The biggest mystery about Conficker is why? What exactly is it that these bad guys are planning to do with it?” Mikko Hyppönen, of the computer security company F-Secure, said.

In the past year the virus has spread to computers in schools, hospitals and government departments. It has got into the defence forces of Britain, Germany and France, grounding the French Navy's fighter jets for a time. Last week a leaked House of Commons memorandum revealed that the parliamentary IT network had been infected.

Microsoft has offered $25,000 (£17,500) for information about Conficker's creator. An alliance of leading computer security experts and internet governance groups has been set up to help to deal with the problem.

The Conficker virus started spreading late last year. At first it was a relatively simple worm, easily dispensed with. However, it has evolved into a more sophisticated and resilient virus that has found new ways to spread. It has also gained the ability to shut down a computer's defences.

It spreads by exploiting a weakness in Windows, the software that runs most computers. At its peak it had infected about 12 million computers, although that may have fallen to about two million with new security measures.

Once the worm is on a computer, that PC becomes part of a “botnet” – a network of computers that can be controlled by the virus's creator.

One popular theory is that the makers are setting up a “computing-for-hire” scheme, where time on infected computers is rented out. Others warn that the makers could try to steal data such as credit card details held in the computers. Experts said that installing a recent version of antivirus software should kill it off.

Rik Ferguson, of Trend Micro, an internet security company, said that the best guess as to who was behind Conficker was a gang based in Ukraine – the first version of the virus was designed not to infect computers there.

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