Fake mobile phone towers, which have previously been reported to exist in the US, are also being used in the UK, Sky News reports.
The towers are used to collect mobile phone communications and will cause serious concern for privacy advocates.
Sky News used bespoke software developed by German security firm GMSK Cryptophone to locate the phone masts, also known as StingRays or IMSI catchers. The broadcaster found 20 occurrences in just three weeks in the London area.
Although StingRays have been suspected to be in use in the country for some time now, this is the first time that technology has been used to prove their existence directly. Deputy director of Privacy International Eric King said that even if the police were using the phone towers to apprehend criminals, phone data from innocent civilians would also be collected.
"With IMSI catchers, it's very difficult for them to be used in a targeted manner,” he explained. "In an urban space, thousands of people's mobile phones would be swept up in that dragnet. What they do with that data, we don't know.”
StingRays work by mimicking the signals sent by genuine mobile phone towers, causing handsets to connect with them. This gives the towers access to sensitive information, such as device identifiers, phone numbers and information regarding phone calls. With some fake towers it is possible to listen to private conversation or intercept text messages.
Despite the evidence collected by Sky News and previous reports by the Times and the Guardian, the Metropolitan Police and the UK’s National Crime Agency refuse to disclose the tactics used to catch criminals, claiming that doing so would be counterproductive.
"Some of what we would like to talk about to get the debate informed and logical, we can't, because it would defeat the purpose of having the tactics in the first place,” said Keith Bristow, director-general of the National Crime Agency.
"Frankly, some of what we need to do is intrusive, it is uncomfortable, and the important thing is we set that out openly and recognise there are difficult choices to be made."