An independent executive branch board has concluded that the National Security Agency’s long-running program to collect billions of Americans’ phone records is illegal and should end.

In a strongly worded report to be issued Thursday, the Privacy and Civil Liberties Oversight Board (PCLOB) said that the statute upon which the program was based, Section 215 of the USA Patriot Act, “does not provide an adequate basis to support this program.”

The board’s conclusion goes further than President Obama, who said in a speech Friday that he thought the NSA’s database of records should be moved out of government hands but did not call for an outright halt to the program. The board had shared its conclusions with Obama in the days leading up to his speech.

The divided panel also concluded that the program raises serious threats to civil liberties, has shown limited value in countering terrorism and is not sustainable from a policy perspective.

“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,” said the report, a copy of which was obtained by The Washington Post. “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.”

The report is bound to spur further debate in an already charged environment in which many lawmakers are divided about the program’s value and legality. Two federal judges have issued conflicting opinions on the program’s constitutionality.

The 238-page report is arguably the most extensive analysis to date of the program’s statutory and constitutional underpinnings, as well as of its practical value.

It rejects the reasoning of at least 15 federal surveillance court judges and the Justice Department in saying that the program cannot be grounded in Section 215. That statute requires that records sought by the government — in this case phone numbers dialed, call times and durations, but not call content — be relevant to an authorized investigation.

But the board found that it is impossible that all the records collected — billions daily — could be relevant to a single investigation “without redefining that word in a manner that is circular, unlimited in scope.” Moreover, instead of compelling phone companies to turn over records already in their possession, the program requires them to furnish newly generated call data on a daily basis. “This is an approach lacking foundation in the statute,” the report said.

“At its core, the approach boils down to the proposition that essentially all telephone records are relevant to essentially all international terrorism investigations,” the report said. This approach, it said, “at minimum, is in deep tension with the statutory requirement that items obtained through a Section 215 order be sought for ‘an investigation,’ not for the purpose of enhancing the government’s counterterrorism capabilities generally.”

The board, which was established at the urging of the 9/11 commission, was not unanimous on the issue of ending bulk collection. Two members concluded that the program, if modified to include additional privacy protections, should continue. The two were Rachel L. Brand and Elisebeth Collins Cook, who served in the Justice Department in the George W. Bush administration. The three members who urged an end to the program are Chairman David Medine, a former Federal Trade Commission official in the Clinton administration; James X. Dempsey, a public policy expert with the privacy group, the Center for Democracy & Technology; and Patricia M. Wald, a retired federal appeals court judge named to the bench by President Jimmy Carter.

The report concluded that the NSA collection raises “constitutional concerns’’ with regard to U.S. citizens’ rights of speech, association and privacy. “The connections revealed by the extensive database of telephone records gathered under the program will necessarily include relationships established among individuals and groups for political, religious, and other expressive purposes,’’ it said. “Compelled disclosure to the government of information revealing these associations can have a chilling effect on the exercise of First Amendment rights.”

The board’s recommendation to end the program goes further than that of a presidentially appointed review panel, which last month urged that the NSA database be shifted out of government hands but that the government find some way to preserve the NSA’s capabilities. That report left open the possibility that a third party or the phone companies could be asked to hold the data.

The PCLOB, by contrast, clearly opposed any legal mandate on the companies to hold data for longer than they do now. And it opposed having a third party hold the data. But both boards concluded that even without the current NSA program, the government would still be able to seek phone records directly from the companies through traditional court orders. The PCLOB said the government could use national security letters in counterterrorism probes.

In its assessment of the program’s value, the board scrutinized 12 terrorism cases cited by the intelligence community that involved information obtained through the Section 215 program. Even in cases where the data related to contacts of a known terrorism suspect, in nearly all of them the benefits were minimal--”generally limited to corroborating information that was obtained independently by the FBI,” the report said.

The board rejected the contention made by officials from Obama on down that the program was necessary to address a gap arising from a failure to detect an al Qaeda terrorist in the United States, Khalid al-Mihdhar, prior to the 2001 attacks. Mihdhar was in phone contact with a safehouse in Yemen, and though the NSA had intercepted the calls, it did not realize at the time that Mihdhar was calling from San Diego.

“The failure to identify Mihdhar’s presence in the United States stemmed primarily from a lack of information sharing among federal agencies, not of a lack of surveillance capabilities,” the report said, noting that in early 2000 the CIA knew Mihdhar had a visa enabling him to enter the United States but did not advise the FBI or watchlist him. “...This was a failure to connect the dots, not a failure to connect enough dots.”

Second, the report said, the government need not have collected the entire nation’s calling records to identify the San Diego number from which Mihdhar made his calls. It asserted that the government could have used existing legal authorities to request from U.S. phone companies the records of any calls made to or from the Yemen number. “Doing so could have identified the San Diego number on the other end of the calls,” though, it noted, the speed of the carriers’ responses likely would vary.

The board also stated that the program played no role in disrupting the 2009 plot to bomb the New York City subway. That case is often cited in discussions of the program’s utility.

“The Board believes that the Section 215 program has contributed only minimal value in combating terrorism beyond what the government already achieves through these and other alternative means,” the report said. “Cessation of the program would eliminate the privacy and civil liberties concerns associated with bulk collection without unduly hampering the government’s efforts, while ensuring that any governmental requests for telephone calling records are tailored to the needs of specific investigations.”