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The campaigning group Privacy International, which is trying to lift the lid on the espionage establishment in the U.K. and the U.S., has filed a legal challenge with the European Court of Human Rights (ECHR) over the British government’s refusal to give up documents relating to a major international spy pact.

The Five Eyes or UKUSA agreement between the U.K., U.S., Australia, New Zealand and Canada, allows the spy agencies of these Anglophone countries to act in close partnership. Privacy International (PI) filed freedom of information requests to get documents that detail how information is shared between the agencies – when and how people can be spied upon, what safeguards exist to combat misuse of data — but Government Communications Headquarters (GCHQ), the British equivalent of the NSA, has rejected every one.

“GCHQ invoked a blanket exemption that excuses it from any obligation to be transparent about its activities to the British public,” PI said in a statement. “The same exemption was also invoked by the agency when Privacy International asked for mundane information such as GCHQ’s cafeteria menu.”

The campaigning group says this blanket exemption falls foul of Article 10 of the European Convention on Human Rights, which guarantees the right to receive information, among other things.

PI Deputy Director Eric King said in the statement that the documents were “critical to proper scrutiny of the spy agencies,” and their release was a “crucial first step” towards the restoration of public trust in those agencies, following Edward Snowden’s NSA and GCHQ revelations.

The ECHR is already dealing with a separate case about the U.K.’s spying activities, brought about by a coalition of activist groups called Privacy Not Prism. That case is in turn very similar to PI’s main case against the U.K. government, which aims to challenge the legality of the PRISM and Tempora spy programs.

The main PI case is currently crawling through the Investigatory Powers Tribunal (IPT), a rather secretive court. PI spokesman Mike Rispoli told me that, if the group loses its IPT challenge and is forced to appeal to the ECHR on that front as well, the PI and Privacy Not Prism cases might possibly be merged. This case about the Five Eyes documents would remain separate though, as it covers different though related matters.