In a ruling that served as a significant check on the powers claimed by Mr Bush to prosecute the War on Terror, the justices ruled that the controversial military trials at Guantanamo breached the Geneva Conventions and US law.

The ruling did not demand the release of the Guantanamo inmates, or the closure of the prison. But it forces the Bush Administration to try the 70 terrorist suspects it has either charged or plans to charge under a new tribunal system that conforms with international law.

Some Republican senators conceded that the ruling also increased pressure on the Administration to repatriate many of the 440 detainees still held at Guantanamo, and who are unlikely to be charged, to their home countries.

Yesterday’s case centred on an appeal by Salim Ahmed Hamdan, Osama bin Laden’s former driver and bodyguard, who claimed that Mr Bush had overstepped his authority by ordering that he be tried under a military tribunal without the protections afforded military prisoners under the Geneva Conventions or US courts.

Mr Bush has argued vociferously that the Geneva Conventions are irrelevant to captured al-Qaeda suspects because they do not belong to a national army, but are instead a deadly new category of killers who themselves do not operate under the normal rules of war.

Under the Bush Administration’s military commission system outlawed yesterday, the defendants would not have had access to much of the prosecution evidence, and would only have had a right of appeal to another military panel on the Guantanamo base.

In a stinging rebuke to the Administration, the Supreme Court, in a 5-3 decision, ruled that the military tribunal convened to try Hamdan “lacks power to proceed because its structures and procedures violate the Geneva Conventions”. The court also said that the tribunal breached the Uniform Code of Military Justice, which governs US courts martial, where military defendants are given far greater rights than they would have had at Guantanamo.

Hamdan’s lawyers claimed victory. “All we’ve wanted is a fair trial and we thank the Supreme Court for ensuring that he will get one, either in a federal court or a military court martial,” said Lieutenant Commander Charles Swift, his lead defence attorney.

Zachary Katznelson, a lawyer for 36 Guantanamo inmates, said: “It’s a fantastic victory for us. It’s a strong rebuke from the Supreme Court to President Bush. They clearly have said he is not above the law and that the men at Guantanamo absolutely have rights, and the military commissions are just blatantly illegal.”

But the ruling also meant that bin Laden’s former driver, and the 70 other Guantanamo inmates facing proceedings, were plunged back into a legal limbo that could still take years to resolve. Hamdan’s case must now be re-argued before the same appeals court that ruled on the matter in 2004, while the Administration must come up with a new system to try the suspects that conforms with yesterday’s ruling.

Mr Bush, appearing at a White House press conference with Junichiro Koizumi, the Japanese Prime Minister, soon after the ruling, said that he would conform with the Supreme Court’s demands. But he insisted that some at Guantanamo, whom he has previously called cold-blooded killers, “need to be tried.”

Two weeks ago, on his return from Baghdad, Mr Bush said that he wanted to close Guantanamo eventually and was waiting on the Supreme Court’s verdict. “Now that the Supreme Court has issued its decision the President should make good on his promise and close Guantanamo,” said Anthony Romero, executive director of the American Civil Liberties Union. A spokesman for Amnesty International, the human rights group, said: “The US Administration should ensure that those held in Guantanamo should be either released or brought before civilian courts.”

In reality, the Bush Administration faces great difficulty in how to release the remaining inmates it does not charge. Most are from Saudi Arabia and Pakistan, whose governments have privately made clear to the White House that they they do not want them returned.

The ruling brought dissent from the court’s three most conservative members. Justice Clarence Thomas read part of his dissent from the bench — something he had never done in his 15 years on the Supreme Court. He said that the decision would “sorely hamper the President’s ability to confront and defeat a new and deadly enemy”.

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