A new American court ruling in favour of Osama bin Laden's driver has cast doubt on the validity of Omar Khadr's war crimes convictions, legal experts said Wednesday.

Even so, they said, several factors make it essentially impossible for Khadr to have his convictions before a military commission in Guantanamo Bay set aside.

Those factors include his guilty pleas, his waiver of his appeal rights, and the murky legal nature of his most serious crime: murder in violation of the law of war.

However, the ruling could potentially help him win parole in Canada.

In their decision on Tuesday, a three-judge panel of the United States Court of Appeals for the District of Columbia Circuit threw out Salim Hamdan's 2008 conviction for providing material support for terrorism.

In essence, the court ruled no such crime existed under international law of war at the time of his alleged offence, and the Military Commissions Act under which he was convicted did not authorize retroactive prosecutions.

Providing material support for terrorism was one of five charges Khadr admitted to as part of a plea agreement in October 2010.

"The logic of the [Appeal Court] overturning this one charge certainly can be argued to apply to all the other charges," said Prof. David Glazier with Loyola Law School in Los Angeles.

However, applying the reasoning to Khadr's most serious offence is less straightforward because prosecutions for murder in violation of the law of war have long existed under international law.

Law experts weigh in

The issue is whether Khadr's admitted conduct — throwing a hand grenade in Afghanistan in July 2002 that killed an armed American special forces soldier — fit the existing bill.

"Killing a combatant with a grenade is pretty central non-violation of law-of-war conduct," said Madeline Morris, an international law expert and professor at Duke Law School in Durham, N.C.

"It's not any murder in violation of the law of war that was ever recognized."

To get around that snag, critics argue, the Military Commissions Act under which Khadr was tried created a new offence to retroactively criminalize what he'd done based on his lack of a uniform.

"All five of the charges against him essentially boil down to the same thing: they boil down to the idea that he was a civilian, was not a 'privileged belligerent,' and was not allowed to participate in hostilities," Glazier said.

"The majority view among law-of-war scholars is that it's not a war crime for an unprivileged belligerent to participate in hostilities. Even if he did in fact do everything he was charged with — including throwing the hand grenade — it's not a war crime."

Ruling could help with parole

Still, Khadr did sign away his appeal rights. Also, the terms of his transfer to Canada preclude attacking his sentence in Canadian courts.

At the same time, the Hamdan decision could bolster arguments that he should be released on parole at the earliest opportunity. Those grounds include his mistreatment in U.S. custody and Ottawa's violation of his rights.

During his confinement at Guantanamo Bay, the Toronto-born Khadr did challenge the validity of the military commission charges — to no avail.

His lawyers later opted for the 2010 plea agreement that capped his sentence at a further eight years and allowed his return to Canada to serve most of it rather than rely on appealing a guilty finding and likely life sentence.

Now 26, Khadr was finally transferred to Canada last month and is currently incarcerated at a maximum security facility in eastern Ontario.

His current lawyers, John Norris and Brydie Bethell, refused to discuss the possible impact of the Hamdan ruling.