More than 20 “intrusive” fake mobile phone towers that eavesdrop on public conversations have been found active in the UK, the first time the technology has been detected in the country.

The IMSI catchers, also known as Stingrays, have been found to be operating in London, but the Metropolitan Police have refused to say who is controlling them or what is being done with the information they are gathering.

IMSI stands for International Mobile Subscriber Identity – a unique number that identifies users on their phone network.

The controversial surveillance technology, used by police forces around the world, is supposedly for catching criminals’ communicating by intercepting information on its way to the network.

It tricks mobile phones into thinking the Stingrays are phone masts, so that handsets connect to the tower and all the data flowing through them is collected – but the masts are unable to distinguish between criminals and everyone else.

A Sky News investigation located the masts using technology made by GSMK Cryptophone, a German security company, and found more than 20 of the rogue towers in three weeks.

Met Commissioner Sir Bernard Hogan-Howe would not confirm or deny that his force was using the technology, telling the channel that “the only people who benefit are the other side, and I see no reason in giving away that sort of thing”.

He said: “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.”

Stingrays are frequently used in the US by police to monitor suspects, though the use of them is inevitably subject to heated debate as they can eavesdrop on anyone’s calls, even without a warrant. The American Civil Liberties Union has called the towers “incredibly invasive”.

Scotland Yard was said to have bought some of the IMSI towers in 2009 and began using them last November, according to reports, although it is the first time evidence has been found that they are operational. Keith Bristow, the director-general of the National Crime Agency, said: “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.”

Eric King, deputy director of Privacy International, said it was time police forces stopped pretending the IMSI towers didn’t exist, so the public could understand the legal framework behind them. He said: “This spying tool has featured in everything from The Wire to Zero Dark Thirty. Companies are selling them on the grey market to anyone who can pay. The only thing we don’t know is what the police are doing to protect people from their use by criminals, and when they use them, what legal frameworks ensures they’re properly used?

“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.”