The Investigatory Powers Tribunal (IPT) will hear from lawyers representing the government and its associated spy agencies including GCHQ on its powers to capture, store and share citizens' data as a legal challenge by civil liberty groups kicks off today.
The complainants are Liberty, Privacy International and Amnesty International, as well as seven overseas human rights groups.
The challenge has been brought against the intelligence agencies in respect of alleged interception activity involving UK and US access to communications, in documents revealed by former National Security Agency (NSA) contractor Edward Snowden.
One of these was a document that revealed GCHQ's Tempora programme, which according to The Guardian, was built up over five years by attaching "intercept probes" to transatlantic fibre-optic cables where they land on British shores.
Other documents suggested that data was passed on from the UK to the NSA through the Prism surveillance programme.
The civil liberties groups believe that there should be a legal framework that is publicly available, in order to hold the government accountable for its actions.
The IPT can investigate claims against MI5, MI6 or GCHQ, and in this case it will seek to determine whether the Tempora and Prism programmes exist - the UK government has neither confirmed nor denied the existence of either programme - and whether either violates articles 8 and 10 of the European Convention on Human Rights.
Despite the UK government's lack of comment in regards to the programmes, Charles Farr, the director general of the Office for Security and Counter Terrorism, openly stated in his witness statement to the tribunal that communications data that has been shared from foreign agencies has led directly to the prevention of terrorist attacks and serious crime, and the saving of lives.
He said he believes that Regulation of Investigatory Powers Act 2000 (RIPA) interception is a "critical tool in investigations into the full range of threats to national security". RIPA dictates which government bodies can and can't legally intercept communications, and there is an ongoing review of the act.
This is just one of many cases that have been brought to court since Snowden leaked thousands of documents to the media.
One investigation which suggested that a fair and balanced independent review was not always possible was the Partliament's Intelligence and Security Committee (ISC)'s investigation of the Prism programme.
It claimed that allegations of GCHQ illegally tapping global internet traffic and phone calls, and sharing it with US counterpart, the National Security Agency (NSA) were "unfounded" and that GCHQ had not circumvented or attempted to circumvent UK law.
The ISC is made up of parliamentarians selected by the Prime Minister, it oversees the intelligence and security activities of the UK, including the policies, expenditure, administration and operation of the MI5, MI6 and GCHQ, among other bodies in the UK intelligence community.
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