Sky News has found evidence that rogue mobile phone towers, which can listen in on people's calls without their knowledge, are being operated in the UK.
IMSI catchers, also known as Stingrays, mimic mobile phone masts and trick phones into logging on.
The controversial surveillance technology is used by police agencies worldwide to target the communications of criminals.
However, Stingrays also collect the data of all other phones in the area, meaning innocent people's communications are spied on.
"With IMSI catchers, it's very difficult for them to be used in a targeted manner," Eric King, deputy director of Privacy International, told Sky News.
"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," Mr King said.
Sky News used software made by GMSK Cryptophone, a German security company, to look for the tell-tale signs of Stingray activity.
Over three weeks, Sky News discovered more than 20 instances in London.
The CEO of Cryptophone, Bjoern Rupp, said: "The abnormal events that Sky News had encountered can clearly be categorised as strong indicators for the presence of IMSI catchers in multiple locations."
Sky News has published its complete data logs here . This is believed to be the first direct evidence of Stingray use in the UK.
In November, The Times reported that the Metropolitan Police Service, the UK's largest police force, was using Stingray technology, citing anonymous sources.
And according to The Guardian, the Metropolitan Police paid £143,455 for the surveillance equipment in 2009.
Despite repeated Freedom of Information requests, including by Sky News, the Met neither confirms nor denies that the force uses IMSI catchers.
Asked directly about the force's use of stingrays by Sky News, Bernard Hogan-Howe, the Met commissioner and the UK's most senior police officer, said: "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.
"If people imagine that we've got the resources to do as much intrusion as they worry about, I would reassure them that it's impossible."
Keith Bristow, the director-general of the National Crime Agency, also told Sky News: "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.
"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."
The police's refusal to comment on IMSI catchers means the legal framework that governs their use is unclear.
Tim Johnston, a barrister who specialises in the law of surveillance at Brick Court Chambers, told Sky News: "Because it's neither confirmed nor denied, we simply don't know on what basis they are being used - if they are being used.
"We don't know how they're being overseen. There are a whole suite of commissioners that oversee communications, that oversee surveillance, and because we don't know the statutory basis that's being relied on, as a consequence we don't know who - if anyone - is overseeing that use."
IMSI catchers are nowadays available to buy on the internet for around £1,000. This raises the possibility that they might be by used foreign governments, private enterprises, or criminals to steal UK citizens' personal data.
Because the police neither confirm nor deny that they use stingrays, it is impossible to say exactly who is operating the stingrays detected by Sky News.