The US military's system of tribunals at Guantánamo Bay was thrown into chaos today after a military judge threw out all charges against a young Canadian detainee.

One senior military official said the ruling in the case of Omar Khadr could have a "huge impact" on the controversial tribunals at the US navy's detention centre in Cuba, the Associated Press reported.

The judge said Mr Khadr - who was captured in Afghanistan as a suspected Taliban fighter in 2002 when he was aged 15 - did not meet the definition of those subject to trial under the new laws in effect at the tribunals.

Army Colonel Peter Brownback, the tribunal judge, said a military review board had labelled Mr Khadr an "enemy combatant" during a 2004 hearing in Guantánamo.

However, the Military Commissions Act adopted by the US Congress in 2006 said only "unlawful enemy combatants" could be tried in the Guantánamo tribunals.

Today Col Brownback said Khadr did not meet that strict definition. "The charges are dismissed without prejudice," the military judge said before adjourning the proceedings.

Shortly afterwards, the chief of military defence lawyers at Guantánamo Bay, Marine Colonel Dwight Sullivan, said the ruling had "huge" impact, because none of the remaining some 380 Guantánamo detainees has been found to be an "unlawful" enemy combatant.

Col Sullivan said the ruling was further evidence the tribunal system was a failed experiment. "We don't need any more evidence that it's a failure," he said. "This system should just stop."

Today's turn of events at the tribunal is the latest legal setback for the Bush administration over the detainees at the prison camp on the southern tip of Cuba.

The current tribunal system was devised after the US supreme court struck down a previous system as unconstitutional last June.

The administration has long been criticised for the legal scenario surrounding the hundreds of suspected terror suspects it has detained there. For years, the detainees were in a legal limbo without apparent prospect of being charged or tried, and the Bush administration has pushed the new tribunals as an antidote to criticism on that front.

It was immediately unclear how US officials would react to this latest legal setback.

Col Brownback gave prosecutors 72 hours to appeal his ruling, though it was not immediately clear whom they could appeal to.

While Col Brownback dismissed the charges, he left open the possibility that they could be re-filed if Mr Khadr went back before a review board and was formally classified as an "unlawful enemy combatant". Mr Khadr has been accused of killing a US soldier with a grenade and wounding another in a battle at a suspected al-Qaida compound in Afghanistan in 2002. During his capture he was shot three times and is nearly blind in one eye as a result of his injuries.

He was also charged with conspiracy and providing material support for terrorism actions such as murder, attempted murder and spying, for allegedly conducting surveillance of US military convoys in Afghanistan.

Col Brownback had rejected a prosecution argument that Mr Khadr clearly met the definition of an "unlawful" combatant because he fought for al-Qaida, which was not part of the regular, uniformed armed forces of any nation.

The prosecution also said it was prepared to produce a video of Mr Khadr wearing civilian clothes while planting a roadside bomb, as evidence he was an unlawful combatant.

The Toronto-born terror suspect had faced a maximum sentence of life imprisonment.

Amnesty International has strongly criticised the decision to prosecute Mr Khadr for murder.

Speaking in April the campaign group's UK director, Katie Allen, said: "To have held a 15-year-old boy in the harsh and lawless conditions of Guantánamo for five years has already been a travesty of justice - and to put him before an unfair 'military commission' trial simply adds to a disgraceful record in his case."

She called on US authorities to transfer his case to a civilian federal court on the US mainland.

Col Sullivan said the ruling also cast doubt on the legality of Australian prisoner David Hicks's guilty plea to supporting terrorism - since Hicks also had not been declared an "unlawful enemy combatant".

He pleaded guilty in March to a charge of providing material support for terrorism and was sent home to Australia to serve a nine-month sentence.

Hicks' conviction was the first at a US war crimes trial since the second world war.