WASHINGTON, June 29: The US Supreme Court ruled on Thursday that President George W. Bush had overstepped his authority in creating military war crimes trials for Guantanamo Bay detainees.

The 5-3 verdict is seen in Washington as a rebuke to the administration’s effort to keep terror suspects out of US judicial system.

The ruling places President Bush in a difficult position because he has said on several occasions that he is awaiting the Supreme Court’s verdict before deciding the proper forum to try Guantanamo prisoners.

Justice John Paul Stevens wrote the ruling, which said the proposed trials were illegal under US law and the Geneva Convention.

A Guantanamo detainee, Salim Ahmed Hamdan, had started the legal proceedings that led to the court declaring the Guantanamo military tribunals illegal. Hamdan, who, according to US authorities, was a driver for Osama bin Laden in Afghanistan, had challenged the military trials as illegal.

The US media projected the case as one of the most significant cases since World War II because it sought to define presidential war powers regarding war prisoners.

Hamdan’s lawyers had challenged two basic principles on which the Bush administration had based its strategy for dealing with terror suspects in its custody: the president’s power to create military tribunals for trying the detainees and his decision that the Geneva Convention for war prisoners does not apply to terror suspects.

The court rejected the government’s position on both the points. It agreed with the petitioner that the president did not have the power to create the tribunals and that the detainees are covered by the Geneva Convention, and therefore, rules governing US court-martial should be applied to them.

The vote was split 5-3, with moderate Justice Anthony M. Kennedy joining the court’s liberal members in the ruling. Chief Justice John Roberts was sidelined in the case because as an appeals court judge, he had backed the government over Hamdan.

Thursday’s ruling overturned that decision.

After the Sept 11 attacks, President Bush established special war crime tribunals for trying prisoners held at the US military base at Guantanamo.

Of about 450 prisoners at Guantanamo, only Hamdan and nine others face charges before a tribunal.

Human rights groups have criticized the tribunals, formally called military commissions, for being fundamentally unfair.

The ruling was eagerly awaited by Bush administration officials, who want to bring charges against more prisoners, and by groups like Human Rights Watch, which has called for closure of Guantanamo. The ruling, however, did not address broader issues such as whether the base should be closed.