A federal judge ruled on Monday that the National Security Agency’s PRISM program, which collects telephone records and metadata and was revealed this summer by whistle-blower Edward Snowden, could be unconstitutional.

U.S. District Court Judge Richard Leon, appointed by former President George W. Bush, ruled that PRISM likely violates the Fourth Amendment’s ban on unreasonable search and seizure. Leon argued that the NSA’s widespread collection of telephone calls made in or to the U.S. represents an “arbitrary invasion” of the lives of its own private citizens. He also said that the Justice Department has done little to demonstrate that these information-collection efforts actually prevent future terrorist acts.

“Plaintiffs have a very significant expectation of privacy in an aggregated collection of their telephone metadata covering the last five years, and the NSA’s Bulk Telephony Metadata Program significantly intrudes on that expectation,” Leon wrote in the ruling. “I have significant doubts about the efficacy of the metadata collection program as a means of conducting time-sensitive investigations in cases involving imminent threats of terrorism.

“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,” he added.

PRISM, believed to have been established in 2007 after former President George W. Bush signed the Protect America Act into law, is an NSA program that has, since its inception, collected data on nearly every telephone call made to, from or in the United States.

Leon issued his ruling in response to a lawsuit levied against the NSA and President Obama by conservative activist Larry Klayman. In the ruling, Leon issued a preliminary injunction ordering the NSA to desist its metadata collection efforts, though he stayed the order and allowed the NSA to submit an appeal.

As Politico notes, this is the first significant legal ruling against the NSA’s PRISM program, which was revealed in June and has been upheld by judges on the Foreign Intelligence Surveillance Court.