Symantec has cautioned that, during the first half of 2005, nearly a third of the world's bot-infected computers (known as zombies) were located in the UK. These are systems that have been hijacked by hackers. The high pace of broadband uptake is one reason for the problem.

Symantec estimates that there are between 1 and 2 million computers worldwide infected with such software, which allows a system to be surreptitiously remote-controlled by hackers.

"Bot networks are valuable for a couple of reasons: One, because they allow for extremely rapid propagation, and two, because they provide a relatively high level of anonymity for providing attacks," Turner said.

On average more than 10,000 bot-infected machines were active each day during the first six months of the year, an increase of more than 140 per cent over the preceding six months, according to Turner.

Typically many computer users do not secure themselves properly, a problem exasperated by the uptake of broadband. Unsecured surfers are online longer with broadband and thus they become more likely to be infected.