Civil liberties campaigners began a legal challenge Monday against the alleged use of mass surveillance programmes by the British intelligence services, in what they said were historic public hearings sparked by the Snowden leaks.
Liberty, Amnesty International and Privacy International have made a claim along with seven foreign human rights groups at the Investigatory Powers Tribunal, which hears complaints about the MI5, MI6 and GCHQ security agencies.
They are challenging the British government's assertion that alleged operations involving the bulk interception, collection, analysis and use of communications data, and the sharing of it with the United States, is lawful.
Ministers refuse to confirm or deny the existence of the British "Tempora" surveillance programme or the extent of their cooperation with the US "Prism" programme, which were revealed by former US intelligence analyst Edward Snowden.
As a result, this week's hearings will be conducted on the basis of what Privacy International called "agreed hypothetical facts", namely that if these programmes exist, whether they would contravene human rights laws.
A spokeswoman for the Foreign Office said it did not comment on ongoing challenges made in the tribunal.
But she said: "The use of interception powers is subject to very strict controls and oversight. The UK's interception regime is entirely compatible with the European Convention on Human Rights."
Campaigners said it was the first time that British government agencies have appeared in a public hearing to answer direct allegations and state their position on mass surveillance.
The heads of the foreign spy agency MI6, the domestic intelligence service MI5 and electronic listening station GCHQ used an unprecedented televised parliamentary hearing in November to deny Britons were under mass surveillance.
The other groups involved in bringing the case are the American Civil Liberties Union, Pakistan's Bytes for All, the Canadian Civil Liberties Association, the Egyptian Initiative for Personal Rights, the Hungarian Civil Liberties Union, the Irish Council for Civil Liberties, and the Legal Resources Centre in South Africa.
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