The 5-3 decision was a frontal assault on Bush's tactics and a reaffirmation of the court's own role in America's system, in which power is shared among three branches of government.

"What it says is that the court has a viable interest in remaining the ultimate authority on the law," said Charles Rose, a constitutional law professor at Stetson University College of Law in Gulfport, Florida.

Other administration anti-terror programmes, including a warrantless-eavesdropping programme that worries even some Republicans, "are based on the same interpretation of presidential authority in a time of war" rejected in the Guantanamo case, says Rose.

Politically, the ruling comes at a bad time for Republicans. It puts Bush allies back on the defensive in a congressional election year over secretive war-on-terror initiatives. It could also help offset the recent momentum from political progress in Iraq.

Bush immediately pledged to work with Congress "to have a military tribunal to hold people to account" that would meet the court's objections. But politicians, legal scholars and Bush's own advisers have not suggested getting Congress to go along would be an easy task.

"This is not a decision that lends itself to a very quick disposition," said White House press secretary Tony Snow.

Snow said the president had not set out to increase his powers, but rather was caught between the Constitution and his obligations to the nation as commander-in-chief. He said the war on terror was "raising questions that are fairly new and people are wrestling with".

The court ruled on Thursday that Bush had overstepped his authority in setting up military tribunals for war-crimes trials at the US prison camp at Guantanamo Bay on Cuba. The justices cited violations of US and international standards on prisoner rights.

Legal and presidential scholars saw the decision as a check on the president's assertion of expanded wartime powers.

They likened it to the court's 1952 rejection of President Harry Truman's efforts to take over a strike-closed steel mill by claiming its steel production was necessary to the US war effort in Korea. Similarly, the Supreme Court rejected President Richard Nixon's claim in the early 1970s of broad wartime power to authorise warrantless wiretap surveillance of domestic groups opposed to the Vietnam War, such as the Black Panthers.

"The Supreme Court normally does not reverse the president during wartime," said Stephen Wayne, a Georgetown University professor and presidential expert. "But this president has claimed a lot of power, much of it under the guise of the state of war that we are in against terrorists. And I think what the Supreme Court is saying is that you've gone too far."

The president and Vice President Dick Cheney have aggressively defended anti-war programmes that have been criticised by Democrats, human-rights groups and many legal scholars.

They have argued that increased surveillance of Americans and the holding of detainees without charges were part of inherent expanded powers that all presidents receive during times of war.

In recent days, they also have criticised the news media, particularly The New York Times, for disclosing once-secret programmes, including the use of an international database to track financial transactions.

"They advanced the notion that things they were doing to fight terror - things that others would view as extreme - were not only appropriate, but that to challenge them was unpatriotic," said Norman Ornstein, a political analyst with the American Enterprise Institute.

"Now this ruling should make it harder to do that - and it certainly puts Republicans more on the defensive over the tactics being used."

Thursday's ruling emboldened administration critics. Senator Patrick Leahy of Vermont, the top Democrat on the Senate Judiciary Committee, called it "a triumph for our constitutional system of checks and balances".

It was a rare rebuke to the president by the court that, after all, put him in office in the contested 2000 presidential race - and which many Democrats claim is moving too far to the political right.

Legal scholars saw little evidence in Thursday's ruling that suggested any political drift by the court one way or the other.

"What it does suggest is that the court is not passive and is vigilant in affirming its own role," said Harold Krent, dean at the Chicago-Kent College of Law. "It suggests that the court will not roll over to assertions of presidential power, at least when its own interests are being threatened."

Tom Raum has covered Washington, DC, for the Associated Press since 1973.

Tom Raum, AP