The trio - a 63-year-old from England, a 28-year-old from Scotland and a 19-year-old from Finland - are accused of creating trojan viruses attached to spam emails which opened a back door allowing access to commercial information on infected PCs.

It is believed that once activated, the virus - known variously as Ryknos, Breplibot or Stinkx - would infect linked machines, creating a zombie network (or botnet) of compromised computers under the gang's control.

Police have yet to disclose the names of companies which were attacked, saying only that "thousands" of machines were hijacked, mostly in the UK.

The three men were arrested in co-ordinated raids at their homes this morning. They were being interviewed today on suspicion of conspiracy to commit unauthorised modification of computer material, a crime covered by the 1992 Computer Security Act.

The charge carries a maximum penalty of six months in prison or a £5,000 fine.

Detective Constable Bob Burls, of the Metropolitan Police Computer Crime Unit, said that the men co-ordinated their activities through a forum which they named Moop - apparently a reference to US cartoon South Park.

"This highly organised group are suspected of writing new computer viruses in order to avoid detection by anti-virus products," he said. "They have been targeting UK businesses since at least 2005, and during this time thousands of computers are known to have been infected across the globe."

According to the CNet website, the Breplibot virus was accompanied by an email specifically tailored to trick business users into executing the attachment.

It read: "Hello, Your photograph was forwarded to us as part of an article we are publishing for our December edition of Total Business Monthly. Can you check over the format and get back to us with your approval or any changes? If the picture is not to your liking then please send a preferred one. We have attached the photo with the article here. Kind regards, Jamie Andrews."

The Finnish National Bureau of Investigation and the Finnish Pori Police Department cooperated in the investigation which has led to a number of computers being seized from their homes.

DC Burls added: "The international co-operation between the specialist law enforcement units has produced this really significant result.

"Today’s arrests will send a clear worldwide signal to the authors of malicious software that national borders will not limit the ability and commitment of law enforcement authorities to clamp down on this criminal activity."

Graham Cluley, a senior technology consultant at internet security firm Sophos, said: "In the past people were writing viruses to show off to friends and colleagues, but these days virus writing has become a much more organised criminal activity.

"Zombie computers can be used by criminal hackers to launch distributed denial-of-service attacks, spread spam messages or to steal confidential information and commit identity theft.

"It seems that this gang the police have cracked in Britain and Finland were doing it for financially-motivated reasons. That is what the Moop group were all about."

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