In what looks to be a major setback for the NSA’s mass surveillance of telephone metadata, U.S. District Judge Richard Leon has ordered the spy agency to stop collecting U.S. citizen’s mobile phone records, and to destroy the files it already holds.

Judge Leon’s ruling comes in the wake of a case brought against the U.S. Government by an individual named Larry Klayman, who sued it following revelations about the NSA’s surveillance from whistleblower Edward Snowden, reports Politico magazine. According to Judge Leon, the NSA’s bulk collection of telephone metadata is unconstitutional, and it should desist immediately. Judge Leon then ordered an injunction preventing the NSA from collecting any more phone records, before ordering it to delete any information collected on the two plaintiffs in the case. However, this decision was immediately stayed pending any appeal the U.S. government might bring.

Judge Leon’s ruling was met with widespread approval from civil liberties groups:

“Congress and the White House have refused to take action to stop the NSA from indiscriminately tracking the phone calls of most Americans, and so our federal courts are our last chance to stop the Obama administration from shredding the Constitution,” said Michael Kieschnick, CEO of CREDO Mobile told SiliconANGLE.

“As the CEO of a mobile phone company, I applaud action by the federal courts to begin to rein in this vacuum-cleaner approach to the private information of innocent Americans.”

Meanwhile, ex-NSA contractor Snowden, now living in Russia, applauded the judge’s decision to issue the injunction.

“I acted on my belief that the NSA’s mass surveillance programs would not withstand a constitutional challenge, and that the American public deserved a chance to see these issues determined by open courts,” Snowden said in a statement to The New York Times.

Metadata reveals too much

Today’s case centers on something called telephone ‘metadata’, rather than actual records of people’s calls. The NSA doesn’t listen in on what people are actually saying, but it does gather information on the people you call, the times of your calls, your location, carrier and more.

Judge Leon said that the case has a historic precedent of sorts, citing the Smith v. Maryland ruling in 1979, noted Ars Technica. In that case, it was ruled that information about the phone numbers people have called is not private, instead being classified as “business records”, hence law enforcement can collect this data without a warrant.

The NSA might have hoped that the same logic would apply to metadata too, but Judge Leon said that wasn’t the case.

“I cannot imagine a more ‘indiscriminate’ and ‘arbitrary invasion’ than this systematic and high-tech collection and retention of personal data on virtually every single citizen for purposes of querying it and analyzing it without judicial approval,” said Judge Leon.

Explaining his decision, Judge Leon said that Smith v. Maryland predates the capabilities of the average smartphone today. The metadata collected by the NSA can tell it far more about a person, their activities, and who they’re connected too, than just the numbers of who they’ve called.

“The main thrust of the government’s argument here is that under Smith, no one has an expectation of privacy, let alone a reasonable one, in the telephony metadata that telecom companies hold as business records; therefore, the Bulk Telephony Metadata Program is not a search,” said Judge Leon in his statement. “I disagree.”

The judge’s ruling was issued just before White House press secretary Jay Carney made his daily press briefing. Carney said he was unaware of the decision and he referred inquiries to the Justice Department, which announced that it’s “reviewing the court’s decision,” reports Politico.