Three computer programmers were arrested on Tuesday in Britain and Finland on suspicion of being involved in an international conspiracy to spread viruses via spam e-mails, British police said.
They are alleged to have targeted UK businesses since at least 2005: thousands of computers are thought to have been infected worldwide.
Police said the "highly-organized" group, arrested in a joint operation between British and Finnish police, are suspected of writing new computer viruses to avoid detection by anti-virus products.
The men are all suspected of being members of a group known online by the name M00P.
Describing the arrests as "really significant," Detective Constable Bob Burls of the Computer Crime Unit at London's Metropolitan Police said the men appeared to be connected via an online company.
"We believe the suspects created and adapted viruses with the aim of causing massive infection by spamming," he added.
The viruses, sent as attachments to spam e-mails, run in the background on an infected computer without the user's knowledge.
Once installed, they allowed the criminals to access any private and commercial data stored on the computer.
A number of computers have been seized at residential addresses in Britain and Finland, as have computer servers used by the suspects.
The suspects are a 63-year-old arrested in Suffolk, a 28-year-old held in Scotland and a 19-year-old held in Finland.