Europe's top rights court ruled Thursday that Britain's programme of mass surveillance, revealed by whistleblower Edward Snowden as part of his sensational leaks on US spying, violated people's right to privacy.
Ruling in the case of Big Brother Watch and Others versus the United Kingdom, the European Court of Human Rights in Strasbourg, France, said the interception of journalistic material also violated the right to freedom of information.
The case was brought by a group of journalists and rights activists who believe that their data may have been targeted.
The court ruled that the existence of the surveillance programme "did not in and of itself violate the convention" but noted "that such a regime had to respect criteria set down in its case-law".
They concluded that the mass trawling for information by Britain's GCHQ spy agency violated Article 8 of the European Convention on Human Rights regarding the right to privacy because there was "insufficient oversight" of the programme.
The court found the oversight to be doubly deficient, in the way in which the GCHQ selected internet providers for intercepting data and then filtered the messages, and the way in which intelligence agents selected which data to examine.
It determined that the regime covering how the spy agency obtained data from internet and phone companies was "not in accordance with the law".
In a further victory for the 16 complainants the court ruled that the programme also provided "insufficient safeguards in respect of confidential journalistic material", violating Article 10 of the European convention, which protects freedom of expression and information.
But it dismissed claims that Britain further violated the privacy of those on whom it snooped by sharing intelligence with foreign governments.
"The regime for sharing intelligence with foreign governments did not violate either Article 8 or Article 10," it said.
Snowden, a former contractor with the National Security Agency of the United States, leaked thousands of classified documents to the press in 2013 which revealed the vast scope of surveillance of private data that was put in place after the 9/11 attacks.
The documents showed, among other things, that Britain spied on foreign politicians at G20 summit meetings in London in 2009 and that its spy agency collected huge quantities of global email messages, Facebook posts, internet histories and calls, and shared them with the NSA.
Snowden, who fled to Russia, is wanted in the US for espionage.
The ECHR noted that national governments enjoy "wide discretion" in deciding what type of surveillance is necessary to protect national security.
But it said such programmes require sufficient oversight to keep the surveillance to what is "necessary in a democratic society".
It noted that some of the data scooped up by the GCHQ "could reveal a great deal about a person's habits and contacts".
The judgement is not final, as it can be appealed to the grand chamber of the Strasbourg court.
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