Were millions of Britons spied on by US government? Surveillance agencies mined personal data of Apple, Google and Facebook to spy on users Britain's secret eavesdropping agency has been spying on UK citizens after cutting a backroom deal with US agents to share personal data plundered from some of the world's biggest internet companies, according to leaked documents. GCHQ has had direct access to the personal information of any Briton with a profile on almost all the mainstream social media sites since 2010 - courtesy of America's National Security Agency. The sensational disclosure came after it emerged the NSA has been pulling individual data from the mainframes of nine top US tech giants, including Facebook, Google and Apple, as part of a top-secret initiative codenamed PRISM. But news that the scandal has spread across the Atlantic not only exposes the sinister lengths to which the British Government is prepared to go to spy on its own people but also raises serious questions over which politicians knew of the programme. Sinister powers: Britain's Government Communications Headquarters, based in Cheltenham, Gloucs., is largely responsible for monitoring the phone calls and emails of terror suspects It means any Brit who sent an email, uploaded a photograph, shared a link or watched a video on one of the breached sites in the past six years, may have done so under the watchful eyes of government spies. Indeed, it will also pile pressure on Prime Minister David Cameron to explain how much he knew about the intrusion as he prepares to attend the Bildeberg Conference, a highly-secretive annual meeting that conspiracy theorists believe is where leaders plot world domination. The nine firms infiltrated by the NSA are Apple, Facebook, Microsoft, Google, Yahoo, YouTube, Skype, AOL and PalTalk. While the NSA is by far the biggest surveillance agency in the world, it regularly shares its work its British counterparts through a shady agreement that allows them to share classified information, bypassing official legal channels. Documents, leaked to the Guardian, reveal that the mutual legal assistance treaty generated 197 British intelligence reports last year alone - a 137 per cent increase on the previous 12 months. GCHQ refused to comment directly on the report, but in a statement it insisted that it operated within a 'strict legal and policy framework'. 'GCHQ takes its obligations under the law very seriously,' the statement said. I know what you're doing this summer: The Obama administration defended the order on Thursday, calling it 'a critical tool in protecting the nation from terrorist threats' Our work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorised, necessary and proportionate, and that there is rigorous oversight, including from the Secretary of State, the Interception and Intelligence Services Commissioners and the Intelligence and Security Committee. The revelation - which is the largest anti-terror intelligence-gathering operation since 9/11 - will place massive pressure on Obama, who is already reeling from the recent IRS scandal. Set up by Harry Truman (pictured) in 1953, The NSA is the eyes and ears of America across the globe, intercepting 1.7 billion emails, phone calls a day. It is the the secretive body that serves the military and intelligence communities by collecting all forms of foreign communications to prevent attacks on the US. It was prohibited by law from intercepting domestic communications without a warrant until George W. Bush issued a caveat in the wake of 9/11 under the controversial 'terrorist surveillance program'. Nonetheless, over the years the NSA has been engulfed in a number of snooping scandals. They include President Nixon's illegal wiretapping, through the NSA, of five members of his national security staff, two newsmen, and a staffer at the Department of Defense in a bid to uncover who was leaking information about his plans for the Vietnam War. In 2005 it was revealed George W Bush had authorized the NSA to eavesdrop on Americans calling abroad without warrants in a bid to thwart terrorism. He strenuously denied the allegations until he finally conceded he had committed an impeachable offense. And in 2009, under President Obama, the US Department of Justice acknowledged the NSA had gone beyond its remit in tapping the phonelines of American citizens, including a Congressman, but claimed that the acts were unintentional and had since been rectified. Last month, it was accused of building an £800million cyber base to keep tabs on American citizens. The state-of-the-art data centre in the Utah desert - codenamed Bumblehive - is intended to bolster online security efforts. But former employees say it could be used to monitor people's private emails. The NSA branded the allegations 'unfounded', adding that it remained 'unwavering' in its respect for U.S. laws and American citizens' civil liberties, and noted that it was subject to broad oversight by all three branches of government. The NSA is known to collaborate with four other allies: Britain, Australia, New Zealand and Canada. Together, they are known as the 'five eyes', of which the second biggest is Britain's General Communications Headquarters (GCHQ). In addition to the names already on the list, the cloud-storage service Dropbox was described as 'coming soon' to PRISM. All forms of wiretapping of U.S. citizens by the NSA requires a warrant from a three-judge court set up under the Foreign Intelligence Surveillance Act passed in 1978. But former President George W. Bush issued an executive order shortly after the September 11, 2001, attacks in New York that authorised the NSA to monitor certain phone calls without obtaining a warrant. The warrantless wiretapping programme remained a secret until 2005, when a whistleblower went to the press to reveal the extent of the surveillance. And although the NSA has strenuously denied acting beyond its surveillance powers, groups such as the American Civil Liberties Union (ACLU) have warned that the Cyber Intelligence Sharing and Protection Act (CISPA) - a bill currently passing through Congress - could dramatically increase the amount of personal data that government agencies have legal access to. Tough questions: The scandal will pile pressure on David Cameron to reveal how much he actually knew 'Reprehensible': Director of National Intelligence James Clapper branded the program 'reprehensible' and said it risks Americans' security But Director of National Intelligence James Clapper denounced the disclosure of highly secret documents Thursday and sought to set the record straight about how the government collects intelligence about people's telephone and Internet use. He called the disclosure of an Internet surveillance program 'reprehensible' and said it risks Americans' security. Plundered: The NSA and FBI have been pulling personal data directly from the mainframes of top US tech giants, including Google, Apple, Facebook, Microsoft and Verizon for the past six years as part. Twitter (top left) is conspicuous from its absence from the list