Khadr's lawyer Dennis Edney said Saturday that the Toronto-born 26-year-old was "looking forward" to the appeal which is expected to be filed "very soon."
Khadr has been held in Ontario's maximum security Millhaven Institution since his transfer to Canada last September from Guantanamo Bay, where he had been held for a decade.
He had pleaded guilty before a widely discredited American military commission in October 2010 to five war crimes — among them killing a U.S. special forces soldier — committed as a 15 year old in Afghanistan. He was given a further eight years behind bars.
Edney said the appeal being launched aims to have all those convictions dismissed.
"We are very confident that the military tribunal convictions will be overturned because in our view there are serious questions about the validity of all these convictions," Edney told The Canadian Press.
Although Khadr opted for a plea agreement in 2010, Edney argued his guilty plea may not have too much of a bearing on his appeal.
"If you plead guilty to a charge which is a nullity in war, then the plea is also a nullity," he said.
The case is still likely to be complicated as Khadr did sign away his appeal rights in 2010, but that obstacle too, Edney contended, could be surmounted.
"If the underlying acts weren't crimes, at least not war crimes, then Mr. Khadr's waiver may also be unreliable," he said.
Edney said his team would be filing an appeal first with a U.S. military commission, and then later in the U.S. civil courts if necessary, to overturn all of Khadr's convictions.
The terms of Khadr's transfer to Canada precluded attacking his sentence in Canadian courts.
Working in Khadr's favour, Edney said, are two similar military commission verdicts which American appeal courts have already thrown out after ruling the crimes did not exist under international law of war at the time.
Last October an American appeal court dismissed Osama bin Laden's driver 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 the alleged offence and retroactive prosecutions were not authorized.
In January, the same court threw out the conviction of Ali Hamza al-Bahlul, a Yemeni who was charged with providing material support to terrorism and conspiracy for making propaganda videos for al-Qaida. In that case, however, a U.S. appeals court said earlier this month that it will reexamine the decision.
Nonetheless, Edney said the rulings on those two cases could bode well for Khadr's appeal.
"As the law now stands, based upon two earlier rulings ... where the civilian appeals court overturned the same charges Omar faced, it concluded the charges were not and are not, recognized international law of war charges," he said.
Edney added that he was surprised previous lawyers retained by Khadr hadn't filed an appeal so far.
"One would expect that should have been done as a matter of course, it wasn't," he said. "I took it upon myself to persuade the military defence department to agree that Omar Khadr's case was worthy of an appeal and they agreed."
John Norris, one of the former Canadian lawyers for Khadr, took exception to Edney's statements.
"Dennis Edney has said we did nothing about an appeal for Mr. Khadr and it was left to him to push the issue," Norris said in an email. "This is false..."
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