Fake mobile phone masts that can be used to eavesdrop on telephone conversations without users being aware have been discovered in London by Sky News. IMSI catchers, also known as "stingrays" after a US company that makes such devices, have been widely used in the US for years. They work by sending out a signal that tricks a mobile phone into connecting with the stingray, rather than a legitimate base station, allowing information to be gathered about the device and its conversations by carrying out a man-in-the-middle (MITM) attack.

It has been suspected for some time that stingrays are being used in the UK: back in 2011, The Guardian ran a story to this effect, but the Metropolitan Police refused to comment. A 2014 article in The Times gave details about what is believed to be the legal framework that regulates their use.

As a post from Privacy International explained, following The Times' report, "when someone is targeted by an IMSI Catcher, it is considered a 'property interference' under the Police Act 1997 Part III ... a 'property interference' is designed to regulate the placing of bugs and breaking into someone's home, not mobile phone interception." This approach allows the use of IMSI catcher devices to be bundled up with other kinds of bugging in official reports, which means it is impossible to know exactly how many times they have been deployed.

Freedom of Information (FOI) requests about IMSI catchers, submitted by Privacy International and Sky News, were all refused. Asked by Sky News about the IMSI catchers discovered in London, the Metropolitan Police commissioner, Bernard Hogan-Howe, would only say: "We're not going to talk about it, because the only people who benefit are the other side, and I see no reason in giving away that sort of thing." That's the standard reply for all kinds of surveillance, but is as unsatisfactory here as it is elsewhere.

From a range of evidence, it is clear that the Metropolitan Police and perhaps other UK police forces are deploying stingray devices. Confirming that fact and providing aggregated statistics on their use would not provide criminals with any important information—for example, when and where they will be deployed in future operations. It would, however, bring a degree of accountability to the use of these devices, which is currently missing. That's important, because IMSI catchers are extremely indiscriminate in their eavesdropping.

As Privacy International's Eric King told Sky News: "With IMSI catchers, it's very difficult for them to be used in a targeted manner. 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." Nor will we be able to find out until the UK authorities place the use of these devices on a proper legal footing, along with all the other intrusive surveillance techniques the Snowden leaks have revealed.