Facebook status updates and Twitter posts are being intercepted by the UK Government because they are regarded as external communications from countries based overseas, it has been revealed.
A legal battle with campaign group Privacy International over the level of surveillance by intelligence agencies has led to the disclosure of the legal loophole by Charles Farr, director general of the Office of Security and Counter-Terrorism.
The continuing legal battle was brought by a collection of civil liberties groups as a result of the Edward Snowden revelations last year.
The former National Security Agency (NSA) employee exposed a wide-ranging network of surveillance by government agencies.
In leaked documents, Mr Snowden revealed extensive snooping was being carried out by the US-based NSA, and GCHQ in the UK, including the interception of email and other communications.
The Government's interpretation means intercepting data can be carried out without a warrant under British law, which identifies an "external communication" as "those communications which are both sent and received outside the British Islands, whether or not they pass through the British Islands in the course of their transit."
In a statement, Mr Farr used the example of someone in the UK carrying out a Google search to demonstrate what would constitute an "external communication".
"Google's data centres, containing its servers, are located around the world, but its largest centres are in the United States.
"So a Google search by an individual in the UK may well involve a communication from the searcher's computer to a Google web server, which is received outside the British Islands, and a communication from Google to the searcher's computer, which is sent outside the British Islands. In such a case, the search would correspondingly involve two 'external communications'."
Privacy International says the interpretation "patronises the British people".
Eric King, deputy director of Privacy International, said: "Intelligence agencies cannot be considered accountable to Parliament and to the public they serve when their actions are obfuscated through secret interpretations of byzantine laws."
Earlier this year, the founder of the world wide web Sir Tim Berners-Lee called for a "bill of rights" to protect web users.
The British physicist also criticised the NSA and GCHQ for the level of surveillance.
In a statement, GCHQ said its work is "carried out in accordance with a strict legal and policy framework which ensures that our activities are authorised, necessary and proportionate".
Mr Berners-Lee set up the World Wide Web Foundation to help protect internet users. Foundation chief executive Anne Jellema said: "Based upon the documents published today, it seems the UK's spy agencies are using flimsy legal justification to sidestep the need for individual warrants and feel able to indiscriminately collect and monitor the private social media and web communications of anyone.
"The toxic cocktail of an opaque system and antiquated laws means that we have no idea how many times this has been done, how many people it has affected, or whether the technology companies involved are aware.
"Intelligence agencies need powers to keep people safe, but these powers must be subject to tough checks and balances or they will erode the very fabric of democracy. Bulk data collection by default, with laws made and enforced in secret, can never be acceptable."
The news follows confirmation from mobile network provider Vodafone that secret wires exist within their network that allow for surveillance by the state. The Heartbleed Bug as well as viruses GoZeus and CryptoLocker have also recently made headlines because of their threat to online privacy.
Emma Carr, acting director of privacy campaign group Big Brother Watch, said: "There has clearly been significant damage done to people's expectations that their communications will remain private from the gaze of companies, the Government or law enforcement agencies.
"There is little wonder that people are increasingly worried about their privacy, which will only have been made worse by the Government's unwillingness to have a proper debate about its surveillance capabilities.
"Our surveillance law needs reviewing, oversight needs to be much stronger and greater transparency about how powers are being used would all improve accountability and trust.
"Addressing this would not require legislation and should be addressed by the Home Secretary without delay."