WASHINGTON — An independent federal privacy watchdog has concluded that the National Security Agency’s program to collect bulk phone call records has provided only “minimal” benefits in counterterrorism efforts, is illegal and should be shut down.

The findings are laid out in a 238-page report, scheduled for release by Thursday and obtained by The New York Times, that represent the first major public statement by the Privacy and Civil Liberties Oversight Board, which Congress made an independent agency in 2007 and only recently became fully operational.

The report is likely to inject a significant new voice into the debate over surveillance, underscoring that the issue was not settled by a high-profile speech President Obama gave last week. Mr. Obama consulted with the board, along with a separate review group that last month delivered its own report about surveillance policies. But while he said in his speech that he was tightening access to the data and declared his intention to find a way to end government collection of the bulk records, he said the program’s capabilities should be preserved.

The Obama administration has portrayed the bulk collection program as useful and lawful while at the same time acknowledging concerns about privacy and potential abuse. But in its report, the board lays out what may be the most detailed critique of the government’s once-secret legal theory behind the program: that a law known as Section 215 of the Patriot Act, which allows the F.B.I. to obtain business records deemed “relevant” to an investigation, can be legitimately interpreted as authorizing the N.S.A. to collect all calling records in the country.