A US federal court has halted a trial at Guantanamo Bay of a former driver for al-Qaeda leader, Osama Bin Laden.
Hamdan says he was a driver who was paid a pittance
The judge said it must first be determined whether Salim Ahmed Hamdan, from Yemen, was covered by the Geneva conventions.
The move could stop all further trials at Guantanamo and the US government says it wants the ruling overturned.
The BBC's Rob Watson in Washington says this is a major setback for the Bush administration.
It is the first time a federal court has stopped trial proceedings at Guantanamo - formally called "military commissions".
Mr Hamdan, from Yemen, faces charges of conspiracy to commit war crimes, including attacking civilians, murder and terrorism.
He is one of four men facing trial by a US military commission at the US naval base in Cuba.
His military-appointed lawyer, Lt Cmdr Charlie Swift, argued the military commissions are not legal and should not have jurisdiction in his client's case.
Mr Hamdan, who denies terrorism, should have the chance to contest his status as "enemy combatant" in a US civilian court, his lawyer argues.
The Washington DC court agreed in part, saying Mr Hamdan should not be tried until a "competent" review tribunal had determined if he is entitled to the greater protections conferred within the Geneva Convention upon prisoners of war.
The Guantanamo trials must also meet codes of military justice, District Judge James Robertson said.
But the US justice department said: "We vigorously disagree with the court's decision, and will seek an emergency stay of the ruling and immediately
"By conferring protected legal status under the Geneva
Conventions on members of Al-Qaeda, the judge has put terrorism on the same legal footing as legitimate methods of waging war," said spokesman Mark Corallo.
The US Supreme Court ruled in July that detainees could take legal challenges to their incarceration to American courts.
Review tribunals were then set up to determine whether detainees could be held indefinitely without charge or trial - as "enemy combatants".
But Mr Hamdan was deemed an enemy combatant before the review tribunals were set up.
Also, like lawyers for dozens of other detainees, Lt Cmdr Swift argues the commissions are illegal and should not have jurisdiction.
The commission's presiding officer ordered an indefinite recess on Monday after a note was passed to the officer, apparently telling him of the Washington court ruling halting the proceedings, the Associated Press news agency reported.
Mr Hamdan says he was paid a pittance as Bin Laden's driver in Afghanistan and denies he was a member of al-Qaeda.
His military commission was due to have begun on 7 December, but pre-trial hearings were already taking place.
Mr Hamdan's defence team said it was a "gigantic victory" for his client. One lawyer said it should stop all further trials at Guantanamo Bay.
The Bush administration's detention policies have been perhaps the most consistently controversial aspect of its "global war on terrorism", says the BBC's Nick Childs at the Pentagon.
Human rights groups say detainees are denied basic human rights and justice.
There have also been reports of abuse by interrogators at Guantanamo, though commanders there now say standards have improved.