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A fierce row broke out tonight after the Government admitted using a loophole to monitor Facebook and Google searches because the online accounts are based outside Britain.

It is even okay to track private messages on social media between people in the UK, according to Security and Counter-Terrorism chief Charles Farr, because internet servers in the US were used.

His admission of mass surveillance without a warrant sparked angry reactions from privacy campaigners and MPs.

Former Defence Secretary Tom Watson said: “In the week the PM celebrates the historical legacy of Magna Carta, it shows what a joke our justice system’s becoming.”

British police and security forces need a warrant to intercept UK phone calls and letters based on suspected criminal activity.

But, under the UK’s Regulation of Investigatory Powers Act, external communications can be freely monitored.

And Mr Farr said he regards communications via Facebook, Twitter, Google and Yahoo as “external communications”.

Mr Farr said someone in the UK carrying out a Google search would also be an “external communication”.

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Liberty director James Welch said: “If there was any doubt our snooping laws need a radical overhaul, there can be no longer.”

The secret policy emerged in a submission to the Investigatory Powers Tribunal, which deals with complaints about intelligence services.

Mr Farr said without mass surveillance the Government wouldn’t have “adequate levels of intelligence”.

He was responding to an action by privacy campaigners on the back of US whistleblower Edward Snowden’s revelations of web monitoring by GCHQ spies.

Social media users in UK 31m Facebook 15m Twitter

Facebook has over 31 million UK users and Twitter has 15 million. Most of our servers are based in the US.

Mr Watson added: “Now you have it. Top spies finally admit they spy on all our Facebook habits and even think they can check our Google searches without a warrant.”

Labour MP Paul Flynn, of the Commons Home Affairs committee, said: “It’s an intrusion too far. You can assume you are being watched over, spied on and investigated 24 hours a day.”