WASHINGTON (AP) — A federal judge ruled Monday that the National Security Agency’s bulk collection of phone records violates the Constitution’s ban on unreasonable searches, but put his decision on hold pending a near-certain government appeal.

U.S. District Court Judge Richard Leon granted a preliminary injunction sought by plaintiffs Larry Klayman and Charles Strange, concluding they were likely to prevail in their constitutional challenge. Leon, an appointee of former President George W. Bush, ruled Monday that the two men are likely to be able to show that their privacy interests outweigh the government’s interest in collecting the data. Leon says that means that massive collection program is an unreasonable search under the Constitution’s Fourth Amendment.

The collection program was disclosed by former NSA systems analyst Edward Snowden, provoking a heated debate over civil liberties. The Obama administration has defended the program as a crucial tool against terrorism.

But in his a 68-page, heavily footnoted opinion, Leon concluded that the government didn’t cite a single instance in which the program “actually stopped an imminent terrorist 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 added.

He said was staying his ruling pending appeal “in light of the significant national security interests at stake in this case and the novelty of the constitutional issues.”