President George Bush overstepped his authority with plans to make Guantanamo prisoners stand trial for war crimes, the US Supreme Court has ruled.

But Mr Bush has insisted he will work with Congress to pursue approval to try terror suspects before military tribunals.

His proposed trials are illegal under US law and the Geneva Convention, Justice John Paul Stevens said.

The case focused on Salim Ahmed Hamdan, a Yemeni who worked as a bodyguard and driver for Osama bin Laden.

Hamdan, 36, has spent four years in Guantanamo, facing a single count of conspiring against US citizens from 1996 to November 2001.

Two years ago, the court rejected Mr Bush's claim to have the authority to seize and detain terrorism suspects and indefinitely deny them access to courts or lawyers.

In this follow-up case, the justices focused solely on the issue of trials for some of the men.

Amnesty International has called the ruling "a victory for the rule of law and human rights".

But the ruling will have little effect on the camp's 450 foreign captives, the camp commander said.

"I don't think there's any direct outcome on our detention operation. The impact will be negligible," Rear Adm Harry Harris said.

Lawyers have already criticised the military tribunals as being rigged to ensure conviction.

Faced with growing international condemnation of the camp after three prisoners committed suicide on June 10, President Bush said he wants to empty the detention centre.