More than 20 fake phone masts have been tuning in to mobile phone conversations across the capital, with the police unable to confirm who is operating them.

The Stingray masts, which tap into the communications data of mobiles, are supposedly used to listen in on criminals by intercepting information intended for the network provider.

However, because the masts are unable to differentiate between criminal communications and regular users, they sweep all the information available to them, meaning millions of people are potentially having their telephone data sent to a third party.

According to a Sky News investigation, there are more than 20 currently in use in London, but their origins remain unknown.

Other police forces across the globe are known to use Stingray masts, which operate by tricking phones into thinking they are regular phone masts. They then harvest their data.

Sky’s investigation offers the first evidence of their use in the UK.

Privacy International deputy director Eric King told Sky News it’s difficult to distinguish between important and non-important data.

“With [Stingray] 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.

“We know police have been using them for years, but this is the first time that it’s been shown that they’re being deployed in the UK,” he added.

Police told Sky they could not confirm whether their officers used the technology. Giving away any information would only “benefit the other side,” they said.

“The only people who benefit are the other side, and I see no reason in giving away that sort of thing,” a spokesperson said.

National Crime Agency Director General Keith Bristow told Sky News: “Frankly, some of what we need to do is intrusive, it is uncomfortable, and the important thing is we set that out openly and recognize there are difficult choices to be made.”

Due to the relative cheapness of the technology, the masts could be used by non-official groups or criminals who have purchased the masts on the internet.

Sky News located the masts using trackers made by German security firm GMSK Cryptophone.