The scam always starts the same way: the phone rings at someone’s home, and the caller – usually with an Indian accent – asks for the householder, quoting their name and address before saying “I’m calling for Microsoft. We’ve had a report from your internet service provider of serious virus problems from your computer.”

Dire forecasts are made that if the problem is not solved, the computer will become unusable. The puzzled owner is then directed to their computer, and asked to open a program called “Windows Event Viewer”. Its contents are, to the average user, worrying: they look like a long list of errors, some labeled “critical”. “Yes, that’s it,” says the caller. “Now let me guide you through the steps to fixing it.”

The computer owner is directed to a website and told to download a program that hands over remote control of the computer, and the caller “installs” various “fixes” for the problem. And then it’s time to pay a fee: £185 for a “subscription” to the “preventative service”.

The only catch: there was never anything wrong with the computer, the caller is not working for Microsoft or the internet service provider, and the owner has given a complete stranger access to every piece of data on their machine.

An investigation by the Guardian has established that this scam, which has been going on quietly since 2008 but has abruptly grown in scale this year, is being run from call centers based in Kolkata, by teams believed to have access to sales databases from computer and software companies.