A sketch of Omar Khadr at a hearing at the U.S. naval base in Guantanamo Bay, Cuba, on Jan. 19. A Federal Court judge ruled Thursday that Prime Minister Stephen Harper must immediately demand Khadr's repatriation. (Janet Hamlin/Pool/Associated Press)

Prime Minister Stephen Harper is required to press the United States for the return of Omar Khadr to Canada from Guantanamo Bay to "comply with a principle of fundamental justice," a Federal Court judge ruled Thursday.

Unmoved, Harper said the government may try to overturn the judge's decision on Khadr, who is accused of killing a U.S. army soldier with a hand grenade during a gunfight in Afghanistan in 2002.

"The facts, in our judgment, have not changed," he told MPs during question period. "We will be looking at the decision very carefully and, obviously, considering an appeal."

Justice James O'Reilly ruled in favour of Khadr's charter challenge of the Canadian government's decision not to request his repatriation from the U.S. detention centre in Guantanamo Bay, Cuba.

"The ongoing refusal of Canada to request Mr. Khadr's repatriation to Canada offends a principle of fundamental justice and violates Mr. Khadr's rights," O'Reilly said in his 43-page decision.

"To mitigate the effect of that violation, Canada must present a request to the United States for Mr. Khadr's repatriation as soon as practicable."

Mother worries government won't follow through

Omar Khadr's mother, Maha Elsamnah, said she is pleased by the court's ruling but wonders whether the Canadian government will simply refuse to heed it.

"You want to put your hope somewhere, and if it's the law, it's the most beautiful thing," she said in Toronto. "But if the ruler or the leader of the nation is controlling everything, you don't know who to trust anymore, the law or the government and the politics."

The CBC’s Rosemary Barton, reporting from Ottawa, said the government has long maintained that because of the seriousness of the charges, the Toronto-born Khadr should face military proceedings in the United States.

In Thursday's decision, the judge pointed out that Khadr is the last citizen of any Western country held at Guantanamo. Other countries have repatriated their citizens.

Khadr's lawyers argued the Canadian government was complicit in the detainee's alleged torture and mistreatment while in U.S. custody and obliged under international law to demand his return.

Khadr, now 22, was 15 at the time he was detained in Afghanistan.

Harper has steadfastly refused to get involved.

All Guantanamo prosecutions on hold

Liberal Leader Michael Ignatieff, NDP Leader Jack Layton and Bloc Québécois Leader Gilles Duceppe have sent a letter to Harper and U.S. President Barack Obama calling for Khadr's repatriation and for any evidence against him to be released to Canadian authorities.

"We feel now that he should come home, and we have never objected to the idea that he should face justice for what he's accused of," Ignatieff said in Washington on Wednesday.

"The problem now is that he's been in custody so long and that the evidentiary trail is so screwed up — it's for prosecutors to determine this — but we have I think legitimate questions as to whether Omar Khadr can get a fair trial anywhere for these crimes."

The U.S. and Canadian governments are signatories to a United Nations protocol that states fighters under age 18 are to be considered child soldiers and must be released and helped to reintegrate into society.

'Appealing this decision only delays the inevitable. It serves no purpose.'— U.S. military defence lawyer William Kuebler

Like all Guantanamo prosecutions, Khadr's case is currently on hold pending a review by Obama's administration.

One of Khadr's Canadian lawyers, Nathan Whitling, said he can't feel too optimistic about Thursday's court decision. He said Khadr has won court cases in the U.S. and Canada before, but to no avail.

"I might be forgiven for not being overly optimistic in terms of what relief might result from this decision today," Whitling said.

"There's no doubt this is a step in the right direction … but we've been through this too many times before to start getting giddy every time we get a good court decision."

The reality is that the U.S. government, not a Canadian court, has the final say over Khadr's fate, Whitling said, so Khadr's legal team will be bringing the court ruling to the attention of the Obama administration.

He says he hopes Harper won't go through with an appeal, although he isn't surprised that Harper has already talked of taking such action.

Khadr's U.S. military defence lawyer, Lt. Cmdr. William Kuebler, suggested it would be pointless for Harper to appeal.

"Omar Khadr is coming back to Canada eventually," he told CBC News. "Appealing this decision only delays the inevitable. It serves no purpose.

"After seven years, or almost seven years, I just can’t imagine that the Canadian government would do anything other than ultimately look at this decision and say, 'Okay, the time has come; let’s do the right thing and bring this young man home.' "

There would be no resistance from Washington, Kuebler predicted.

"It’s a tremendous embarrassment, what we’ve done to this young man in terms of detention and interrogation, in terms of fabricating evidence and false allegations against him, and I can’t imagine that the Obama administration would do anything other than jump at the chance to send this young man home if the Canadian government followed this decision and asked for his return."

With files from The Canadian Press