The US Supreme Court today blocked the trial of Guantanamo Bay prisoners in special military courts, ruling that the process drawn up by the Bush Administration broke both American law and the Geneva Conventions.

In a blow to the executive powers of President Bush, America's highest court decided that the first 10 military tribunals of Guantanamo detainees - originally scheduled for later this year - will not go ahead.

The court’s ruling says nothing about whether the prison should be shut, dealing solely with the proposed trials.

The "structure and procedures" of the proposed military commission violated international laws governing the treatment of prisoners of war, it said, essentially, that prisoners should not be tried by their military adversaries.

"Trial by military commission raises separation-of-powers concerns of the highest order," wrote Supreme Court Justice John Paul Stevens in the majority decision. The court ruled against the White House by a narrow margin of 5-3.

The ruling raises major questions about the legal status of the approximately 450 men still being held at the US military prison in Cuba and exactly how, when and where the administration might pursue the charges against them.

It also seems likely to further fuel international criticism of the administration, including by many US allies, for its treatment of detainees at Guantanamo, Abu Ghraib in Iraq and elsewhere.

Conditions at the detention centre were once again the subject of international scrutiny earlier this month when three detainees committed suicide by hanging themselves in their dormitories.

President Bush, in a brief press conference, said he believed that a compromise could be found to allow the tribunals to go ahead, adding that the ruling "won’t cause killers to be put out on the street."

Chief Justice John Roberts, appointed by President Bush to lead the court, did not take part in the decision because he had ruled in an earlier Court of Appeals hearing on the case.

Justice Clarence Thomas, one of the three dissenting judges, wrote a strongly-worded critique of the ruling and took the unprecedented step of reading part of it from the bench. He said the court’s decision would "sorely hamper the President’s ability to confront and defeat a new and deadly enemy".

Today's case was brought by Salim Ahmed Hamdan, a 36-year-old Yemeni who worked as a bodyguard and driver for Osama bin Laden. Hamdan has spent four years in Guantanamo Bay and faces a single count of conspiring against US citizens from 1996 to November 2001.

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