A U.S. appeals court on Tuesday overturned the conviction of Osama bin Laden's former driver and bodyguard, Salim Hamdan, on charges of supporting terrorism, in a long-running case emerging from the American military trials at Guantanamo Bay, Cuba.

The U.S. Court of Appeals for the District of Columbia Circuit concluded that providing support for terrorism was not a war crime at the time of Hamdan's alleged conduct from 1996 to 2001 and therefore could not support a conviction. Human rights activists hailed the ruling as a blow to the legitimacy of the U.S. military commission system.

Hamdan was captured at a roadblock in Afghanistan in November 2001, not long after the U.S. invasion of that country following the September 11 attacks on the United States.

In the first U.S. war crimes trial since World War Two, Hamdan was convicted in August 2008 of providing personal services in support of terrorism by driving and guarding bin Laden, the al Qaeda leader who was killed in a U.S. raid in Pakistan last year.

Hamdan was sentenced to 66 months in prison but given credit for time served at the U.S. detention facility at Guantanamo Bay. He was returned to Yemen in November 2008 and set free in January 2009 to live with his family in Sanaa.

The appeals court found that even though Hamdan already had been released from U.S. custody, the appeal of his conviction was not moot.

At the trial, prosecutors said Hamdan was close to al Qaeda's inner circle while his lawyers asserted that Hamdan was simply a driver and mechanic in the motor pool who needed the $200 monthly salary.

Set Guantanamo precedent

Hamdan won a prior victory in 2006 when the U.S. Supreme Court scrapped the first version of the Guantanamo court system.

After that Supreme Court decision, Congress passed the Military Commissions Act of 2006, which listed a number of specific new war crimes that could be prosecuted by a military commission, including providing support for terrorism. The government refiled charges against Hamdan under that new law.

He was eventually convicted on five counts of providing material support for terrorism.

On appeal, Hamdan argued that Congress lacked the power to make providing support for terrorism a war crime.

The appeals court refused to rule on that question, but it concluded that the military commissions act did not authorize prosecutions for conduct that occurred before the law was passed and that was not prohibited at the time it occurred.

"Because we read the Military Commissions Act not to sanction retroactive punishment for new crimes, and because material support for terrorism was not a pre-existing war crime ... Hamdan's conviction for material support for terrorism cannot stand," Judge Brett Kavanaugh wrote for the three-judge panel.

A Justice Department spokesman said the U.S. government is reviewing the ruling.

Brian Mizer, who was a U.S. Navy lieutenant commander when he served as Hamdan's defence lawyer during his military trial, welcomed the decision.

"After 10 years of litigation, the government effort at GTMO has produced a handful of convictions for war crimes that don't exist, which occurred during a war that may not exist, and were tried before a war court that should not exist," Mizer said in an email, using the military abbreviation for the Guantanamo Bay U.S. naval base.

Mizer, who now works as a public defender in Virginia, did not represent Hamdan in the appeal.

Other prosecutions in question

The United States, during Republican President George W. Bush's administration, set up the special military commissions to handle the trials of foreign suspects held at Guantanamo rather than having the existing military justice system or civilian courts deal with them. The Guantanamo detention facility opened in 2002.

Tuesday's decision strikes a blow to the legitimacy of the military commission system and calls into question prosecutions of offenses that are not internationally recognized war crimes, said Raha Wala, a lawyer with Human Rights First.

To date, most of the military commission cases have been based on charges of material support or conspiracy that were added by the 2006 statute, he said.

"All of these prosecutions are now in jeopardy," Wala said, including the ongoing prosecution of Saudi prisoner Abd al-Rahim al-Nashiri set to resume at Guantanamo next week.

Australian former prisoner David Hicks, who pleaded guilty to providing material support for terrorism, could now ask the U.S. court to overturn his conviction despite waiving his appellate rights as part of the plea deal that allowed him to go home, said Adam Thurschwell, one of Hamdan's appellate lawyers.

"You can't be deemed a criminal if it wasn't a crime," Thurschwell said.

The decision should encourage the federal government to prosecute Guantanamo prisoners in federal court, said Zachary Katznelson, a lawyer for the American Civil Liberties Union. In federal court, "the law is clear and prisoners have the legal rights necessary to ensure trials are fair, transparent, and viewed as legitimate," he said in an emailed statement.

The case is Hamdan v. USA, U.S. Court of Appeals for the District of Columbia, No. 11-1257.