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The rapid growth of broadband has led to an unintended consequence in the U.K.: a swelling in the ranks of the country's botnet armies. During the first half of 2005, nearly a third of the world's bot-infected computers, called "zombies," were located in the U.K., according to security product vendor Symantec.

Symantec estimates that between 1 million and 2 million computers worldwide are infected with this 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," Turner said.

For more information about botnet armies and the dangers and reach of malware, read PCW's exclusive series called "Web of Crime".

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.

Latest Figures

In its biennial Internet Security Threat report, set to be published Monday, Symantec found that London and Winsford, England, were the top two bot-infected cities worldwide, with 8 percent and 5 percent of the world's infected computers, respectively. Seoul came in at third place, with 4 percent.

The U.S. and China were the second-largest and third-largest providers of bot-infected systems, with 19 percent and 7 percent, respectively.

Several other vendors have spotlighted zombie problems in recent months. McAfee estimated significant growth of aggressive zombies in a report earlier this year. America Online was found the most zombie-infected network in another study.

London is considered one of the largest hub cities on the Internet, with a total of 1.1 terabits per second of international bandwidth available in the city, according to the research company Telegeography.

Use of DSL broadband has been growing rapidly throughout the U.K. Earlier this year, U.K. telecommunications giant BT Group announced that it had more than doubled its DSL connections over the previous year's number, reaching a total of 5 million lines 12 months earlier than expected.