A US federal court threw out Tuesday the conviction of Osama bin Laden's former driver, who was jailed at Guantanamo, saying material support for terrorism does not constitute a war crime.

While Salim Hamdan has already been freed, the Yemeni's appeal in civilian court could have ramifications for other suspects as "material support for terrorism" is a common charge against detainees at the US prison camp in Cuba.

A three-judge bench at the US Court of Appeals in Washington said that a law that listed material support for terrorism as a war crime -- approved in 2006 in response to Hamdan's case -- could not apply to him retroactively.

The court said that US prosecutors instead had to rely on international law, which defines some forms of terrorism -- such as the intentional targeting of civilians -- as war crimes.

"But the issue here is whether 'material support for terrorism' is an international-law war crime. The answer is no," wrote Judge Brett Kavanaugh.

"International law leaves it to individual nations to proscribe material support for terrorism under their domestic laws if they so choose. There is no international-law proscription of material support for terrorism," he said.

Kavanaugh -- an appointee of former president George W. Bush who is generally considered a conservative -- also dismissed the argument that the Hamdan case was moot as he had already been released.

According to prosecutors, Hamdan moved from Yemen to Afghanistan -- then coming under control of the hardline Taliban movement -- in 1996 and participated in a training camp of bin Laden's Al-Qaeda network.

Hamdan became a driver who transported weapons and other goods between Al-Qaeda sites in Afghanistan and later became bin Laden's personal driver and bodyguard, according to US court documents.

Hamdan was captured in November 2001, one month after the United States and its allies opened a war in Afghanistan in response to the September 11 attacks carried out by Al-Qaeda.

Prosecutors said Hamdan was captured while driving toward the Taliban's hub of Kandahar and possessed two anti-aircraft missiles.

The United States transferred Hamdan back to Yemen at the end of his sentence in late 2008 and local authorities freed him in January 2009. But US legal rights activists kept up his case in court.