British spies can legally monitor its citizens' Facebook, Twitter and Google activities without a warrant, the U.K.'s top counter-terrorism chief revealed.
Charles Farr, the director-general of the Office for Security and Counter-Terrorism, revealed in legal documents that, under U.K. law, some social media and even email are legitimate targets of surveillance for the government as they are not considered domestic communications.
The disclosure was made in a 48-page document released as part of a legal challenge brought last year from several privacy groups against Britain's spy agencies. The groups, which include Privacy International and Amnesty, claim the mass surveillance activities of the U.S. and Britain revealed by former National Security Agency contractor Edward Snowden were illegal.
Read MoreUK spies to share data with businesses
Farr's statement is the first time the government has admitted to targeting social media and other electronic communications.
The law used by the intelligence agencies is the U.K.'s Regulation of Investigatory Powers Act (RIPA). Under the law, spy agencies must obtain a legal warrant to monitor so-called "internal communications" between British residents located in the country and only if they suspect the person in question of unlawful activity.
But under a more grey area of the law called "section 8.4", authorities do not require such stringent checks and can obtain a general warrant to monitor "external communications".
Using this part of the law, Farr said the government can monitor Twitter, Facebook, Google searches and emails since the information which is being transmitted from the U.K., such as a Google search, is then being sent to servers across the world before returning to Britain.
Read MoreUK, US under fire for Angry Birds spy claims
Farr continually stated that the surveillance methods are crucial to keeping the U.K. safe from terrorists.
The privacy-advocacy groups claim the interception of electronic messages is being done through the government's so-called TEMPORA spy program revealed by Snowden, though Farr said he could not confirm or deny that such a scheme exists.
But Privacy International slammed the government for its tactics.
Read MoreNSA chief: US spy agency saw changed behavior after Snowden
"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," Eric King, deputy director of Privacy International, said in a statement.
The case will be heard by the Investigatory Powers Tribunal on July 14.
A Google spokesperson said the government did not have access to its data.
"We cannot say this more clearly—government does not have access to Google servers...We provide user data to governments only in accordance with the law. Our legal team reviews each and every request, and frequently pushes back when requests are overly broad or don't follow the correct process," Google told CNBC in an emailed statement.
Twitter declined to comment and Facebook did not responded to an email request for comment.