A report by the research arm of Congress says President Bush's rationale for eavesdropping on Americans without warrants conflicts with existing law and hinges on weak legal arguments.
The report by the Congressional Research Service rejects the administration's argument that the president has authority to order the secret monitoring of telephone calls and e-mails between people inside the United States and abroad.
It also rejects the administration's assertion that Congress authorized such eavesdropping following the September 11, 2001 terrorist attacks when it approved a resolution allowing the president to use all necessary force against terrorist groups.
Administration officials have dismissed the report, saying Mr. Bush acted within his constitutional authority. A Justice Department official, Brian Roehrkasse told The New York Times the eavesdropping program is a "critical tool" in the war on terror.
The Congressional Research Service conducts studies at the request of U.S. lawmakers, and its reports are generally seen as non-partisan.
Some information for this report was provided by AP and Reuters.