IMSI catchers are portable surveillance tools used for spying on thousands of phones in a targeted area, tracking their location and even intercepting calls, messages, and data. They are supposed to help identify serious criminals, but cannot operate without monitoring innocent people too.

UK police have IMSI catchers, but they refuse to tell the public how and when they are used. This has privacy campaigners worried. And, even if the state is using them sparingly, what if criminals also have access to the technology?