About 660 people are being held at the base in Guantanamo Bay

The two rulings said the US Government could not detain suspects without granting them access to lawyers.

They show the president cannot step above the law, said American Civil Liberties Union head Anthony Romero.

Amnesty International urged Washington to abandon "wrong" practices following the court decisions.

'Flawed' decision

The first decision, from an appeal court in New York, ruled that Jose Padilla - a US citizen arrested at Chicago airport and accused of being involved in a plot to set off a radiological weapon - cannot be held indefinitely in a military prison.

Mr Padilla has been held in a naval facility in Charleston, South Carolina, since June 2002 as an enemy combatant following a presidential decree.

Mr Padilla is thought to be the only US citizen since World War II to be detained on a presidential order.

The court said that, although it recognised that the US Government had a responsibility to protect the nation, presidential authority "does not exist in a vacuum".

The president cannot step above the law by a stroke of his pen Anthony Romero US Civil Liberties Union Briton faces 'dirty bomb' charge

The judges said Mr Padilla should be released from military custody, but added that the US Government was free to transfer him to civilian jurisdiction.

White House spokesman Scott McClellan called the court's ruling "troubling and flawed" and said the US Justice Department would seek a stay on the court's ruling and request further judicial review.

The decision came shortly after another US federal appeals court in San Francisco, California, ruled that detainees being held by the US military at Camp Delta in Guantanamo Bay, Cuba should have access to lawyers and US courts.

The Guantanamo ruling - relating to the case of a Libyan national captured in Afghanistan - is the first to overrule the US administration's position that the men held there can be detained indefinitely without legal recourse.

Not above the law

"The president cannot step above the law by a stroke of his pen, especially in times of war or national crisis when our system of equal and dispassionate justice is all the more essential," said Anthony Romero, American Civil Liberties Union Executive Director.

Jose Padilla is currently being held in a military jail

International human rights group Amnesty International also stressed the need to respect justice for all.

"[The decision] should serve as a further reminder to the Bush administration that the practice of indefinite detention without charge or trial is wrong and should be repudiated at home as it is abroad," AI's Vienna Colucci said.

The issue of representation in US courts for Guantanamo detainees is already under investigation by the nation's highest legal body, the US Supreme Court.

BBC correspondent Justin Webb in Washington says this judgement from a court known for its hostility to the Bush administration will probably not take effect until the Supreme Court rules next year on the issue and it could be overturned.

But it does set the stage for the possibility that the US legal profession will force legal representation and legal rights to be given to Guantanamo prisoners, our correspondent adds.