The US court of appeals has ruled that the bulk collection of telephone metadata is unlawful, in a landmark decision that clears the way for a full legal challenge against the National Security Agency.

A panel of three federal judges for the second circuit overturned an earlier ruling that the controversial surveillance practice first revealed to the US public by NSA whistleblower Edward Snowden in 2013 could not be subject to judicial review.

NSA bulk data collection ruled illegal – read the court document

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But the judges also waded into the charged and ongoing debate over the reauthorization of a key Patriot Act provision currently before US legislators. That provision, which the appeals court ruled the NSA program surpassed, will expire on June 1 amid gridlock in Washington on what to do about it.

The judges opted not to end the domestic bulk collection while Congress decides its fate, calling judicial inaction "a lesser intrusion" on privacy than at the time the case was initially argued.

"In light of the asserted national security interests at stake, we deem it prudent to pause to allow an opportunity for debate in Congress that may (or may not) profoundly alter the legal landscape," the judges ruled.

But they also sent a tacit warning to Senator Mitch McConnell, the Republican leader who is pushing to re-authorize the provision, known as Section 215, without modification: "There will be time then to address appellants' constitutional issues."

"We hold that the text of section 215 cannot bear the weight the government asks us to assign to it, and that it does not authorize the telephone metadata program," concluded their judgement.