A US court of appeals has overturned a conviction of Salim Ahmed Hamdan, a former driver for Osama Bin Laden.

Tuesday's reversal would see Hamdan's 2008 conviction of supporter terrorism being thrown out because he was tried under a law that did not apply to his the allegations against him.

“Because we read the Military Commissions Act not to retroactively punish new crimes, and because material support for terrorism was not a pre-existing war crime under 10 U.S.C. § 821, Hamdan’s conviction for material support for terrorism cannot stand,” the District of Columbia appellate court ruled.

The court noted that Hamdan’s crime occurred from 1996 to 2001.

The 2006 Military Commissions  Act, which specifically lists material support for terrorism as a war crime triable by military commission, cannot be applied retroactively to cover Hamdan's five-year stint as a driver for the al-Qaeda leader.

Hamdan was sentenced to five-and-a-half years by a panel of six US military officers in 2008.

The ruling found that his role as driver to the al-Qaeda leader qualified as material support for terrorism.

He was released early into his sentence because he had already spent six years in custody at the Guantanamo Bay Naval Base detention camp in Cuba.

Hamdan was the first detainee in the detention camp to be convicted of war crimes. He was captured in Afghanistan two months after the September 11 attacks.