The rapid growth of broadband in the UK has led to an unintended consequence: a massive increase in the number of compromised computers that can be controlled by hackers without the user's knowledge.

During the first half of 2005 nearly a third of the world's computers infected with so-called bot software, known as zombies, were located in the UK, according to Symantec.

The firm estimates that there are between one million and two million computers worldwide infected with bot software, which allows a system to be surreptitiously remote-controlled by hackers, said Dean Turner, senior manager of the Symantec security response team.

"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," he 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 percent over the preceding six months, according to Turner.

These networks of zombie computers have become a weapon of choice for spammers and phishers, as well as attackers looking to swamp a victim's server with a flood of unwanted data, a technique called a distributed denial of service attack.

In its biennial Internet Security Threat report, set to be published today, Symantec found that London and Winsford were the top two bot-infected cities worldwide, with eight and five percent of the world's infected computers respectively. Seoul came in at third place, with four percent.

The US and China were the second and third-largest providers of bot-infected systems, with 19 percent and seven percent respectively.

London is considered one of the largest hub cities on the internet, with a total of 1.1Tbps (terabits per second) of international bandwidth available in the city, according to the research company Telegeography. One terabit is equal to a million megabits.

Use of broadband has been growing rapidly throughout the UK. Earlier this year BT announced that it had more than doubled its DSL (digital subscriber line) connections over the previous year's number, reaching a total of five million lines 12 months earlier than expected.