A scheme involving bulk collection of telephone metadata has been ruled unlawful by the US Court of Appeals, in a landmark judgement.
A panel of three federal judges overturned a ruling that said the programme, which was exposed by NSA whistleblower Edward Snowden, could not be put before judicial review.
Their verdict clears the way for a fuller legal challenge against the American spy agency, and could considerably alter the cybersecurity landscape around the world.
It a statement the trio said: "We hold that the text of Section 215 [the clause in the Patriot Act previously thought to enable metadata collection] cannot bear the weight the government asks us to assign to it, and that it does not authorise the telephone metadata programme.
"Such a monumental shift in our approach to combating terrorism requires a clearer signal from congress [the US equivalent of parliament] than a recycling of oft-used language long held in similar contexts to mean something far narrower.
"We conclude that to allow the government to collect phone records only because they may become relevant to a possible authorised investigation in the future fails even the permissive ‘relevance’ test."
The ruling comes just weeks before Section 215 lapses on June 1, an expiration which has prompted senator Mitch McConnell, who leads the majority in the upper house of the Senate, to campaign to reapprove it without change.
Since the start of this year civil liberties groups have been campaigning to stop the section being reapproved, though as the judges acknowledged the court verdict will considerably alter the debate.
Rand Paul, a Republican running for the next US presidential election partly on a platform of opposition to government snooping, tweeted his support for the verdict.
"The phone records of law abiding citizens are none of the NSA’s business!" he said. "Pleased with the ruling this morning."
Yesterday it was revealed that the NSA had created a ‘google for voice’ for searching phone calls.