WASHINGTON -- The Supreme Court dealt the Bush administration a devastating legal loss in the war on terrorism Thursday, ruling that the president overstepped his constitutional authority by creating ad hoc military tribunals for prisoners at Guantánamo Bay, Cuba.

The ruling is the third high-court rebuke of the president's claim that he has nearly unlimited power to determine the fate of detainees captured in the war on terrorism. The decision throws plans for the 450 or so foreigners at the prison at the U.S. naval base into indefinite limbo.

The 5-3 decision, written by Justice John Paul Stevens with an important concurrence by Justice Anthony M. Kennedy, essentially said the tribunals violated U.S. law and the Geneva Conventions of 1949 because they didn't provide the safeguards that either civilian or military courts required.

The ruling overturned an appeals court judgment that the tribunals were lawful.

The case, Hamdan v. Rumsfeld, involves Salim Ahmed Hamdan, a 36-year-old Yemeni whom the government believes was Osama bin Laden's driver and bodyguard in Afghanistan. He's been in jail since 2002, when he was captured in Afghanistan and sent to Guantánamo.

"The military commission at issue lacks the power to proceed because its structure and procedures violate both the [Uniform Code of Military Justice] and the four Geneva Conventions signed in 1949, " Stevens wrote.

"Even assuming Hamdan is a dangerous individual . . . the executive nevertheless must comply with the prevailing rule of law in undertaking to try him and subject him to criminal punishment."

The justices pointed out that Congress could have authorized the president to conduct extemporaneous proceedings, but it didn't, neither in the Sept. 18, 2001, bill to authorize military force in response to the 9/11 terrorist attacks or in a 2005 law that prevented Guantánamo detainees from appealing their status to federal courts.

BUSH WILL COMPLY

President Bush said he would comply with the ruling and work with Congress to devise some form of tribunal that met the court's legal standard. He also said the ruling wouldn't derail his efforts to keep the nation safe.

"The American people need to know that this ruling, as I understand it, won't cause killers to be put out on the street, " he said. "I understand we're in a war on terror; that these people were picked up off of a battlefield; and I will protect the people and, at the same time, conform with the findings of the Supreme Court."

Senate Majority Leader Bill Frist, R-Tenn., said he would introduce legislation to authorize military commissions to try terrorist combatants after Congress returned from its July 4 recess.

"Since this issue so directly impacts our national security, I will pursue the earliest possible action in the United States Senate, " the senator said.

The Senate Armed Services Committee plans hearings on how to structure tribunals later this summer. Senate Judiciary Committee Chairman Arlen Specter, R-Pa., introduced a bill Thursday that would define terms and processes.

Some legal experts, however, questioned whether cases involving Guantánamo detainees would hold up in courts governed by any U.S. legal code, civil or military.

"The government has a very difficult mountain to climb, " said Michael Ratner, the president of the Center for Constitutional Rights, which is involved in challenges to the detentions. "They're not going to be able to use evidence that has been coerced from detainees. What's the government going to have left for evidence?"