Three computer experts have been arrested over an alleged international plot to spread viruses via email.
Police say the viruses run without the knowledge of the computer owner and allow criminals to access any stored private and commercial information.
The three men are alleged to have targeted UK businesses since at least 2005, and infected computers worldwide.
Those arrested are a 63-year-old in Suffolk, a 28-year-old in Scotland and a 19-year-old in Finland.
They are suspected of being members of an online group called M00P, delivering the viruses as attachments on unsolicited emails known as spam.
The arrests follow a joint operation between the Metropolitan Police and officers in Finland.
Detective Constable Bob Burls, of the Met Police's computer crime unit, said: "The international co-operation between the specialist law enforcement units has produced this really significant result.
"We believe the suspects created and adapted viruses with the aim of causing massive infection by spamming."
He said he hoped the arrests would send a "clear signal" that national borders would not stop the authorities taking action over malicious computer software.
Police have raided residential addresses in both the UK and Finland and seized a number of computers and computer servers.