(Inserts charges against Saudi, paragraph 5)

MIAMI, Dec 20 (Reuters) - Osama bin Laden's driver is not a prisoner of war as defined by the Geneva Conventions and can be tried by a Guantanamo war crimes tribunal, a U.S. military judge ruled in a decision made public on Thursday.

The judge said Yemeni prisoner Salim Ahmed Hamdan is an "unlawful enemy combatant" under the law passed by Congress last year to provide a legal basis to try non-Americans on terrorism charges in a special war crimes court at the U.S. naval base in Guantanamo Bay, Cuba.

Lawyers for Hamdan, who acknowledge he earned $200 a month driving and guarding the al Qaeda leader in Afghanistan, said he was a civilian support worker who should be considered a prisoner of war deserving of the safeguards outlined in the Geneva Conventions that govern the treatment of war captives.

POWs can be tried by court-martial but not in ad hoc tribunals such as those still evolving at Guantanamo. Deeming Hamdan a POW would have raised questions about the status of many of the other 285 men held at the Guantanamo prison camp, most for more than five years.

Only about five of the prisoners have been charged under the revised Guantanamo tribunal system created in 2006, including a Saudi charged on Thursday with plotting with al Qaeda to blow up a ship.

The ruling by the military judge, Navy Capt. Keith Allred, clears the way for Hamdan's trial in the Guantanamo war court, where he faces life in prison if convicted of conspiracy and providing material support for terrorism.

It will be the third attempt to try him after earlier charges were thrown out by the U.S. Supreme Court and Allred.

The widely criticized Guantanamo court has yet to see a trial completed. The lone conviction came in a negotiated guilty plea for an Australian now nearing the end of a nine-month prison term in his homeland.

The Bush administration has long argued that the alleged al Qaeda and Taliban operatives at Guantanamo are not POWs because they are not members of the uniformed army of any nation.

CAPTURED NEAR KANDAHAR

Allred said Hamdan's lawyers had shown no evidence he was part of any army, militia or volunteer group granted "lawful combatant" status under the Geneva treaties, or that he played a support role for any such group.

He said U.S. military and federal agents had shown credible evidence Hamdan enthusiastically worked for bin Laden after learning he directed the bombing of U.S. embassies in Africa and other attacks, and that he drove the al Qaeda leader to help him elude capture after the Sept. 11 attacks.

Hamdan was captured near Kandahar in Afghanistan on Nov. 24, 2001. Prosecutors said he was driving toward a battlefield in a car carrying two anti-aircraft missiles without the firing mechanisms. The judge said that showed Hamdan was participating in hostilities against the United States and its allies, since their aircraft were the only local targets for the missiles.

Allred also rejected an argument that the Guantanamo court's jurisdiction was retroactively based on a flawed administrative hearing that labeled Hamdan an enemy combatant three years ago.

The judge said his finding was based on a December hearing he conducted in the presence of journalists and human rights monitors, in which Hamdan had six lawyers, called witnesses and confronted those who testified against him.

"The commission concludes, then, that he is an alien unlawful enemy combatant, and not a lawful combatant entitled to Prisoner of War protection," Allred wrote.

Hamdan called another Guantanamo captive to testify in his defense but was refused permission to call others, including Sept. 11 mastermind Khalid Sheikh Mohammed, on his behalf. (Editing by Patricia Zengerle)