LONDON — A London-based advocacy group filed suit over America's international data dragnet Monday, a complaint which one expert says could have significant ramifications for the future of U.S.-British intelligence-sharing.

In a complaint filed with the Investigatory Powers Tribunal, Britain's interception watchdog, Privacy International alleges that the National Security Agency and its British counterpart GCHQ are spying on one another's citizens and swapping the intercepted information without proper oversight. The complaint also accuses GCHQ of overstepping British law through the mass monitoring of U.K communications.

"It is a fundamental breach of the social contract if the government can operate with unrestrained power in such an arbitrary fashion," Privacy's Eric King said in a statement.

Over the past month, both the NSA and GCHQ have seen details of their globe-spanning intelligence-gathering efforts splayed across the pages of the Guardian newspaper and other outlets — leaks which have lifted the curtain on a series of programs apparently aimed at securing access to as big a chunk as possible of the world's communications.

Some in Britain have expressed concerns that GCHQ is drawing on the NSA's massive data pool to dodge restrictions on domestic espionage. In its complaint, Privacy said it was concerned that its own communications had been intercepted by the Americans and subsequently handed to British authorities. It demanded an immediate end to GCHQ's exploitation of NSA-obtained intelligence on British residents and an injunction against the blanket interception of U.K. data over fiber optic cables.

GCHQ declined to comment on the suit. U.K. officials have repeatedly insisted their spies work within the law.

Legal experts say Privacy's complaint faces long odds at Britain's Investigatory Powers Tribunal, where only 10 out of more than 1,000 complaints have ever been upheld. They're divided as to its prospects should it make its way to the European Court of Human Rights — a Strasbourg, France-based, body whose rulings have occasionally frustrated U.K. leaders.

Simon McKay, a British human rights lawyer and an authority on surveillance law, said the failure of British officials to regulate the acquisition of foreign intelligence means it's "almost without question" that they're breach of European human rights law. He said GCHQ could well see itself rebuffed in Strasbourg, something which could lead to a dramatic change in the way Britain swapped intelligence with its American ally.

Ian Walden, a professor of information communications law at Queen Mary, University of London, was more pessimistic, saying the otherwise robust European Court tended to be "quite deferential to member states with regard to national security."

"I don't think this is a slam-dunk," he said.

Privacy's suit is one of several which have been spawned by NSA surveillance revelations.

In the United States, groups including the Electronic Privacy Information Center and the American Civil Liberties Union have said they are suing over the agency's spying. In Britain, civil liberties group Liberty says it is finalizing its own complaint before the Investigatory Powers Tribunal.

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Online:

Privacy International: https://www.privacyinternational.org/

Investigatory Powers Tribunal: http://www.ipt-uk.com/

GCHQ: http://www.gchq.gov.uk/

NSA: http://www.nsa.gov/