Government surveillance agency GCHQ breached human rights legislation in the way it accessed internet data gathered by US National Security Agency, the Investigatory Powers Tribunal (IPT) has ruled.
The tribunal found that the intelligence-sharing relationship was unlawful prior to December 2014, because rules governing the UK's access to the NSA's mass electronic surveillance programmes PRISM and Upstream were secret.
It's the first time the tribunal - which considers complaints brought against GCHQ, MI5 and MI6 - has found against the intelligence agencies according to Liberty, which last year along with Privacy International and Amnesty International challenged GCHQ's surveillance practices in the wake of the Edward Snowden revelations.
In December 2014, the Tribunal held that GCHQ's access to NSA intelligence was lawful from that time onward - because secret policies governing the UK-US relationship were made public during the case.
James Welch, legal director for Liberty, said: "We now know that, by keeping the public in the dark about their secret dealings with the National Security Agency, GCHQ acted unlawfully and violated our rights."
And Eric King, deputy director of Privacy International, said: "Today's decision confirms to the public what many have said all along -- over the past decade, GCHQ and the NSA have been engaged in an illegal mass surveillance sharing program that has affected millions of people around the world.
GCHQ noted the judgment reaffirmed the IPT's main December ruling which found that its bulk interception regime and intelligence sharing regime, were fully compatible with human rights, in particular the right to privacy. It said the judgment focuses primarily on "a discrete and purely historical issue" about whether those legal frameworks were also fully compatible at a point before these legal proceedings began.
The agency said the judgment "does not in any way suggest that important safeguards protecting privacy were not in place at all relevant times" and does not require GCHQ to change what it does to protect national security in any way.
However, Liberty said it disagreed that the safeguards revealed are sufficient to make GCHQ's mass surveillance and intelligence-sharing activities lawful, and plans to challenge the Tribunal's December decision at the European Court of Human Rights.
A GCHQ spokesperson said: "By its nature, much of GCHQ's work must remain secret. But we are working with the rest of government to improve public understanding about what we do and the strong legal and policy framework that underpins all our work. We continue to do what we can to place information safely into the public domain that can help to achieve this".