A judge has ruled that the US National Security Agency's gathering of Americans' phone records is likely unlawful.
The judgement on the so-called metadata counter terrorism programme was made yesterday by District Judge Richard Leon, appointed by Republican President George W Bush in 2002, raising "serious doubts" about the value of the scheme.
"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," he wrote in a 68-page ruling.
The Department of Justice said it was reviewing the ruling in a case brought by Larry Klayman, a conservative lawyer, and Charles Strange, described in court documents as the father of a cryptologist technician for the NSA who was killed in Afghanistan in 2011.
The judge ordered the government to stop collecting data about the two plaintiffs, who were Verizon Communications customers, though Leon suspended enforcement of his injunction against the program "in light of the significant national security interests at stake in this case and the novelty of the constitutional issues" pending an expected appeal by the government.
An official said an appeal was likely and Verizon declined comment.
"We believe the program is constitutional as previous judges have found," Department of Justice spokesman Andrew Ames said in a statement.
Leon expressed scepticism of the program's value, writing that the government could not cite a single instance in which the bulk data actually stopped an imminent attack.
"I have serious doubts about the efficacy of the metadata collection program as a means of conducting time-sensitive investigations in cases involving imminent threats of terrorism," he wrote.
That is important, he added, because for the program to be constitutional, the government must show its effectiveness outweighs privacy interests.
In defending the data collection, Justice Department lawyers have relied in part on a 1979 ruling from the Supreme Court that said people have little privacy interest when it comes to records held by a third party such as a phone company. Leon wrote that the latest circumstances were different.
"The government, in its understandable zeal to protect our homeland, has crafted a counter terrorism program with respect to telephone metadata that strikes the balance based in large part on a 34-year-old Supreme Court precedent, the relevance of which has been eclipsed by technological advances and a cell phone-centric lifestyle heretofore inconceivable," he wrote.
Documents provided by former NSA contractor Edward Snowden showed that a US surveillance court had secretly approved the collection of millions of raw daily phone records in America, such as the length of calls and the numbers that are dialled.
Snowden, in a statement sent by journalist Glenn Greenwald, applauded the ruling.
"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," he said.
"Today, a secret program authorised by a secret court was, when exposed to the light of day, found to violate Americans' rights. It is the first of many."
Greg Nojeim, senior counsel at the Center for Democracy and Technology, a non-profit group in Washington, said the ruling "means that the NSA bulk collection program is skating on thin constitutional ice."
A committee of experts appointed by the Obama Administration to review NSA activities is expected to recommend that the spy agency give up collection of masses of metadata and instead require telephone companies to hold onto it so it can be searched. But intelligence officials and the phone companies themselves are said to oppose such a plan.