The mass interception and surveillance of UK citizens’ activity on Facebook, Twitter, YouTube and Google is legal, the government’s top anti-terror chief has said.

In the first detailed defence of the UK’s surveillance policies since the Snowden revelations, Charles Farr, the director general of the Office for Security and Counter-Terrorism, has said that the surveillance of such popular sites is legal because their US origins means they count as “external communications”.

In a 48-page statement issued in response to a legal challenge brought by Privacy International, Liberty, Amnesty international and seven other civil liberties groups, Farr admits that the government allows the interception of a massive range of online activities without a warrant.

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It was previously thought that the interception of communications within the country was covered by section 8(1) of the Regulation of Investigatory Powers Act (RIPA), with warrants granted when law enforcement suspected the individual in question of illegal activity.

However, by defining these web services as “external communications," they fall under the general warrants of section 8(4) of RIPA. This means that a range of activities – from emails to Facebook messages to Google searches – can all be intercepted even when the police have no grounds to suspect the individuals of wrongdoing.

Farr argues that the convoluted paths that data can take across the internet justifies an indiscriminate approach to data collection: “The only practical way in which the government can ensure that it is able to obtain at least a fraction of the type of communication in which it is interested is to provide for the interception of a large volume of communication.”

Referring to the concern that analysts would therefore be able to read the private communications of law abiding citizens, Farr said: "The analyst, being only human and having a job to do, will have forgotten (if he or she ever took it in) what the irrelevant communication contained."

Eric King, deputy director of Privacy International, said: “The suggestion that violations of the right to privacy are meaningless if the violator subsequently forgets about it not only offends the fundamental, inalienable nature of human rights, but patronises the British people, who will not accept such a meagre excuse for the loss of their civil liberties.”

James Welch, Legal Director of Liberty, said: “The security services consider that they’re entitled to read, listen and analyse all our communications on Facebook, Google and other US-based platforms. If there was any remaining doubt that our snooping laws need a radical overhaul there can be no longer.”