UK's intelligence organisation GCHQ says the country's espionage rules allow it to routinely intercept online communications of the country's Facebook, Twitter and Google users.
A written statement by the Office for Security and Counter-terrorism chief Charles Farr said interception of tweets, posts and YouTube searches is permitted by law as they are classified as "external communications."
GCHQ has been bestowed with broad powers to intercept communications outside the country, but requires a warrant and suspicion of wrongdoing to monitor Britons.
Britain's Home Office confirmed the report, which was made public in response to a legal action by civil liberties groups who are seeking to put an end to cyber-spying.
The office said the legal terms of the policy are defined in the regulation of Investigatory Powers Act, which covers "common factual scenarios involving the use of the Internet, such as a Google search, a search of YouTube for a video, a tweet on Twitter, or the posting of a message on Facebook.
Farr however commented that though the agency intercepts most of communications this way, they "cannot be read, looked at or listened to" except in strictly limited circumstances, terming it as a "significant distinction."
He justified the surveillance saying that even though the messages are sent by and intended for Britons, they travel through servers hosted in America. However, he did mention that emails sent between two people in Britain would usually be classified as internal even if they are sent by a route outside the country.
But civil liberties organisations say the rules were too vague and allowed for mass surveillance.
Liberty legal director James Welch said, "The security services consider that they're entitled to read, listen to and analyse all our communications on Facebook, Google and other US-based platforms. "
The legal action was initiated by the rights groups after leaks about cyber-snooping surfaced from former US National Security Agency (NSA) whistleblower Edward Snowden.
Snowden revealed details of a program called Prism in US that grant the NSA access to internet companies' customer data, and a British operation, Tempora, that allows GCHQ to access data from undersea cables.
Farr neither commented on Tempora nor confirmed whether GCHQ had received information from Prism.
But GCHQ said it would start sharing classified cyber threat information with private companies in the wake of concerns over increasingly sophisticated targeting of businesses by hackers.
Privacy International was sharp in its criticism of GCHQ's alleged surveillance methods, arguing that it is a gross violation of individuals' privacy.
But Farr says that such accusations are exaggerated, and the surveillance that is conducted is necessary to protect national security.
The report also stated that the UK has for "many years faced a serious threat from terrorism," the principal threat of which "continues to derive from militant Islamist terrorists."