An independent government privacy watchdog has declared that the NSA bulk phone metadata program, used to collect the phone records of virtually all Americans, is illegal and should end, as it has had "minimal" impact on terrorism investigations.

These findings, included in a 238-page report by the Privacy and Civil Liberties Oversight Board (PCLOB), are yet another blow to perhaps the most controversial NSA program revealed by top secret documents leaked by Edward Snowden. The report is scheduled for release on Thursday, but it was leaked to the New York Times and The Washington Post.

See also: 5 Things Obama Failed to Address in the NSA Speech

The NSA bulk phone metadata program "implicates constitutional concerns under the First and Fourth Amendments, raises serious threats to privacy and civil liberties as a policy matter, and has shown only limited value," the report stated. "As a result, the board recommends that the government end the program."

In the report, the PCLOB concluded that Section 215 of the Patriot Act, which lets the U.S. government request businesses to turn over various records during a terrorism investigation, "does not provide an adequate basis to support this program." Section 215 has been referred to by the NSA and the Obama administration as the legal underpinning of the program.

The American Civil Liberties Union, a staunch critic of the NSA's surveillance program, applauded the report.

"The NSA’s call-records dragnet is illegal and ineffective and presents a serious threat to civil liberties," said Jameel Jaffer, the ACLU Deputy Legal Director. "The board’s report makes even clearer that the government’s surveillance policies, as well as our system of oversight, are in need of far-reaching reform."

An internal NSA review panel appointed by Obama recommended putting an end to the program, too, as stated in a report published Dec. 18, 2013. But it also posited alternative ways for the government to access the data if necessary, such as by mandating phone companies to retain phone records longer than they do now or creating an independent third party that would keep the database.

The PCLOB report opposed both of these solutions and went even further in critiquing the program, arguing that it did not prove useful in fighting terrorism.

"We have not identified a single instance involving a threat to the United States in which the telephone records program made a concrete difference in the outcome of a counterterrorism investigation," the report read. "Moreover, we are aware of no instance in which the program directly contributed to the discovery of a previously unknown terrorist plot or the disruption of a terrorist attack."

A separate study, published by the New America Foundation last week, reached a similar conclusion, finding the program had an impact in just 1.8% of terrorism cases investigated since the attacks on Sept. 11.

In a long-anticipated speech on Friday, Obama announced a transition to end the NSA bulk phone metadata program as it currently exists. But the exact changes to the program are unclear. Obama also praised the collection program as an essential tool to fight terrorism, and said its capabilities should be preserved.

Obama saw parts of the PCLOB report before his speech, according to the White House, and some of his proposed changes echo its recommendations. The White House said it will review the full report upon its release, but disagreed with its findings regarding the NSA bulk phone metadata program.

"We disagree with the Board’s analysis on the legality of the program," said Caitlin Hayden, a White House spokesperson. "The Administration believes that the program is lawful. As the president has said, though, he believes we can and should make changes in the program that will give the American people greater confidence in it."

The PCLOB is comprised of David Medine, a former Clinton administration Federal Trade Commission official; Patricia M. Wald, a retired federal appeals court judge; James X. Dempsey, a civil liberties advocate specializing in technology issues; and Rachel L. Brand and Elisebeth Collins Cook, both Justice Department lawyers in the George W. Bush administration.

The full report is embedded below.

PCLOB Report on the NSA Bulk Phone Metadata Program

This post was updated with the full report on Jan. 23 at 1 p.m. ET.