The U.S. government is expected to appeal a judge's decision that the National Security Agency's bulk collection of millions of Americans' telephone records is likely unconstitutional, in a case where the Supreme Court will likely have the last word.

U.S. District Court Judge Richard Leon put his decision to grant an injunction against the NSA on hold Monday. He said he was staying the ruling pending appeal "in light of the significant national security interests at stake in this case and the novelty of the constitutional issues."

The collection program was disclosed by former NSA systems analyst Edward Snowden, provoking a heated national and international debate.

Robert F. Turner, a professor at the University of Virginia's Center for National Security Law, predicted Leon's decision was highly likely to be reversed on appeal. He said the collection of telephone metadata _ the issue in Monday's ruling _ already has been addressed and resolved by the Supreme Court.

In his ruling, Leon granted a preliminary injunction against the collecting of the phone records of two men who had challenged the program, and said any such records for the men should be destroyed. The plaintiffs are Larry Klayman, a conservative lawyer, and Charles Strange, the father of a cryptologist technician who was killed in Afghanistan when his helicopter was shot down in 2011. The son worked for the NSA and support personnel for Navy SEAL Team Six.

Leon, an appointee of President George W. Bush, ruled that the two men "have a substantial likelihood of showing" that their privacy interests outweigh the government's interest in collecting the data "and therefore the NSA's bulk collection program is indeed an unreasonable search under the Constitution's Fourth Amendment." The amendment prohibits unreasonable searches and seizures and requires that a search warrant be sanctioned by a judge.

"I have little doubt that the author of our Constitution, James Madison, who cautioned us to beware `the abridgment of freedom of the people by gradual and silent encroachments by those in power,' would be aghast," he declared.

In addition to civil liberties critics, big communications companies are unhappy with the NSA program, concerned about a loss of business from major clients who are worried about government snooping.

President Barack Obama will meet Tuesday with executives from leading technology companies. The meeting had been previously scheduled, but the NSA program is sure to be on the agenda, and now the court ruling will be in the mix.

The Obama administration has defended the program as a crucial tool against terrorism.

But in his 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.

Former NSA Director Michael Hayden challenged Leon's conclusion.

"If part of his constitutional ruling is the intelligence utility of the program, he's not in a good position to judge that," Hayden said in an interview. "He makes a judgment that the government was not able to show that this stopped an imminent terrorist attack. That's not the only metric," as metadata also helps intelligence analysts understand their adversaries and track their networks and behavior.

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Associated Press writers Mark Sherman, Pete Yost, Nedra Pickler and Kimberly Dozier in Washington and Bradley Brooks in Brazil contributed to this report.