GCHQ mounted a mass surveillance exercise which collected information from “every visible user on the internet”, according to documents obtained by the US National Security Agency whistleblower Edward Snowden.

They suggested the Cheltenham-based agency was harvesting data from the users of search engines, social media, online radio, news and pornographic websites.

The classified files appear to uncover the existence of a secret programme called Karma Police, named after a Radiohead song.

This built profiles of individuals by scrutinising their web browsing habits including visits to websites operated by the BBC, Channel 4 News and Reuters, messages on chat forums and activity on Facebook.

It stored “metadata” about people’s emails, texts and phone calls, revealing who they contacted but not what was said or written. The agency also monitored more than 200,000 people who listened to online radio shows, according to the documents.

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The website The Intercept, which received the Snowdon files, said the programme was launched about seven years ago “without any public debate or scrutiny”.

It said that in 2012 GCHQ was gathering some 50 billion metadata records a day about online communications and GCHQ intended to boost its capacity to 100 billion records a day by the end of this year.

The Intercept said the data was held for months in a vast store nicknamed the Black Hole and was sifted by data analysts in a hunt for online activity linked to terrorism or other criminal activity.

Javier Ruiz, the policy director of Open Rights Group, said: “These latest revelations confirm that, despite the Government’s denials, the security services are engaged in mass surveillance and the law is failing to protect our privacy.

A GCHQ spokesman said: “All of GCHQ’s work is carried out in accordance with a strict legal and policy framework, which ensures our activities are authorised, necessary and proportionate.”