The US Supreme Court has agreed to take up the case of Ali Saleh al-Marri, a graduate student and suspected Al Qaeda sleeper agent who has been held without charge in a US military prison for more than five years.

The high court made the announcement on Friday after its private conference.

The case poses the most significant unresolved legal question yet in the Bush administration's controversial approach to the war on terror, and it sets the stage for another potential landmark national security decision next June.

At issue is whether the president has the power to order someone who is legally present in the United States to be held in indefinite military detention as an "enemy combatant."

The Fourth US Circuit Court of Appeals ruled 5 to 4 in July that Congress had given the president the power to declare and hold enemy combatants, including individuals who are legally present in the US.

Lawyers for Mr. Marri are appealing that decision, saying civilians in the US may only be detained through the criminal-justice system. They are asking the nation's highest court to state clearly what the law is and what it requires.

"The lower court has replaced settled and historic protections with confusion," writes Marri's lawyer, Jonathan Hafetz of the American Civil Liberties Union, in his brief to the court. "The Fourth Circuit has cast a pall over the physical liberty of all persons in the United States."

The Marri case will not be argued and decided for many months, well after the departure of President Bush and the arrival of the Obama administration.

Mr. Hafetz says the US has long barred military action against civilians within US borders. Congress has authorized war fighting and the holding of enemy combatants overseas, but lawmakers have insisted that the criminal justice system, not the military, be used to safeguard Americans at home, Hafetz says.

Government lawyers say that shortly after the 9/11 attacks, Congress passed a law called the Authorization for Use of Military Force (AUMF). They say that law granted the president broad power to designate civilians in the US - or anywhere - enemy combatants in the war on terror. Once so designated, they may be held by the military for as long as the war on terror continues, they say.

Marri is no different than Mohammed Atta and the other 9/11 hijackers, government lawyers say. "There can be no serious doubt that Congress, in passing the AUMF, sought to authorize the use of 'all necessary and appropriate force' against aliens who have come to the United States to take an active part in al Qaeda terror operations," writes Solicitor General Gregory Garre in his brief urging the court not to take up the case.

He says the AUMF was written to "protect United States citizens both at home and abroad."

Marri's lawyers argue that the government's power to capture prisoners on a battlefield and hold those prisoners for the duration of hostilities does not apply to civilians in the US, who are far removed from any actual battlefield.

Government lawyers counter that the battlefield in the war on terror extends worldwide, including within US borders. Al Qaeda operatives may be held by the US military regardless of where they are captured, they say.

Marri is a citizen of both Saudi Arabia and Qatar. He has been held in US custody nearly seven years. For the last five years he's been detained without charge as an enemy combatant in a 9-foot by 6-foot cell at the US Naval Consolidated Brig in Charleston, S.C.

US officials suspect Marri was sent to America by Osama bin Laden to carry out a second wave of terror attacks after the 9/11 attacks. They say he was plotting to wreak havoc on the US financial system by hacking into bank and other databases. In addition, he received Al Qaeda training in the use of poisons, according to the government.

Marri's lawyers deny the allegations.

Once in military custody, intelligence officials tried to pressure Marri into confessing. His lawyers say he was subject to harsh interrogation tactics. It is unclear what he told interrogators.

Solicitor General Garre urged the high court to reject Marri's appeal. He said the case should return to a federal judge in South Carolina where Marri would be given an opportunity to fight the allegations against him in a court hearing.

Hafetz says the central issue in the Marri case has been in some stage of litigation for nearly seven years. "The permissible bounds of the executive's domestic detention authority remain shockingly ill-defined," he says.

"The issue before the court is not whether the government's allegations are true," Hafetz says. "It is whether the government must prove those allegations in a criminal trial, with the full safeguards of the criminal process. Further proceedings on remand will not affect or clarify that legal question."

Hafetz says the appeals court decision in Marri's case gives broad authority for the government to hold US citizens in open-ended military detention without charge.

"The Fourth Circuit's decision has profound repercussions," he writes. "It grants the executive discretion to displace the constitutional protections of the criminal justice system, including the right to speedy presentment, confrontation, and trial by jury, merely by alleging a connection to possible terrorist activity."

Garre said the case does not involve whether American citizens can be detained as enemy combatants. He noted that Marri is a foreign national, not a citizen.