A US appeals court has ruled that US President George Bush does not have power to detain an American citizen seized on US soil as an enemy combatant.
The decision could force Mr Jose Padilla, held in a so-called "dirty bomb" plot, to be tried in civilian courts.
In a 2-1 ruling, a three-judge panel sitting in New York, said Mr Padilla 's detention was not authorised by Congress and that Bush could not designate him as an enemy combatant without the authorisation. Padilla is accused of plotting to detonate a "dirty bomb," which uses conventional explosives to disperse radioactive materials.
The former Chicago gang member was arrested in May 2002 and within days was moved to a naval prison in South Carolina.
"As this court sits only a short distance from where the World Trade Centre stood, we are as keenly aware as anyone of the threat al-Qaida poses to our country and of the responsibilities the president and law enforcement officials bear for protecting the nation," the court said.
"But presidential authority does not exist in a vacuum, and this case involves not whether those responsibilities should be aggressively pursued, but whether the president is obligated, in the circumstances presented here, to share them with Congress," it added.