Ottawa's plan to offer former child soldier Omar Khadr and his lawyers a $10.5-million compensation package is facing a complex international legal battle that threatens to delay any payout.

The legal battle is set to be waged by Tabitha Speer, the widow of Sergeant Christopher Speer, an American soldier killed in Afghanistan in the 2002 battle that ended with Mr. Khadr's capture, and Layne Morris, a former U.S. soldier partially blinded by a grenade in that fateful firefight. A Canadian lawyer acting for the pair filed a notice of application in Ontario Superior Court on June 8, seeking an injunction to block any payment of compensation to Mr. Khadr.

Instead, any payment due to Mr. Khadr should be redirected to them and their families, their application says. Their court filing demands that Ontario's courts recognize the $134-million (U.S.) default judgment the plaintiffs won in a U.S. federal court in Utah against Mr. Khadr on June 8, 2015, for his alleged actions in the 2002 battle in Afghanistan, when he was just 15.

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Their application in Ontario Superior Court comes exactly two years later after the Utah ruling, as the legal time limit for seeking to collect debt in Ontario was about to expire. The plaintiffs maintained when they won their judgment in 2015 that they planned to file actions to collect on it in Canada.

But the latest legal filing has only come to light after The Globe and Mail reported this week that Mr. Khadr – who is suing the federal government over Canadian involvement in his mistreatment while detained in the U.S. military prison at Guantanamo Bay – is in line to receive an apology and a $10.5-million compensation package from Ottawa. The settlement was supposed to be unveiled this week.

No court date has been set for a hearing on Ms. Speer and Mr. Morris's application. Their Canadian lawyer, David Winer, declined to speak to The Globe. Their Salt Lake City-based lawyer, Donald Winder, did not respond to requests for comment.

Mr. Khadr's long-time lawyer, Edmonton-based Dennis Edney, said he was not aware of the court application. (According to the court file, it had not yet been served on Mr. Khadr or his lawyers.) But Mr. Edney said any attempt to redirect compensation owing to Mr. Khadr could result in a major legal battle.

"I don't understand what basis in international law that they have in being able to sue Omar Khadr for the death of Christopher Speer on a battlefield, when there is absolutely no evidence that he did [what is alleged] other than Omar Khadr's own admitting to it while being tortured in a place that is renowned for torture," said Mr. Edney, who would not otherwise comment on the reports of a settlement for his client. "... They are going to have quite a fight."

University of Western Ontario law professor Stephen Pitel, an expert on questions of jurisdiction and the recognition of foreign judgments by Canadian courts, said that although Canada's rules are "pretty liberal," getting this Utah ruling recognized here could be an uphill battle.

Prof. Pitel said the plaintiffs will need to show how the Utah court had proper jurisdiction over both a battlefield incident in far-off Afghanistan and a defendant who did not show up to court because he was in prison. Mr. Khadr's lawyers could also fight the recognition by arguing that it offends the principle of "natural justice" or Canadian "public policy."

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But Prof. Pitel said Ms. Speer and Mr. Morris may be helped by a recent Ontario Court of Appeal ruling, released just last week, if it stands. In a sprawling case known as Tracy v. Iran (Information and Security), Ontario's highest court upheld a lower court decision and sided with Americans seeking to enforce U.S. judgments against Iran and demanding compensation for victims of Iranian-sponsored terrorism.

"There are a lot of legal issues here, and it is going to take a while to unravel them," Prof. Pitel said.

As part of a plea deal in 2010, Mr. Khadr pleaded guilty to Sgt. Speer's murder and was returned to Canada in 2012, where he served time in a maximum-security prison before he was freed on bail in 2015. He later recanted that he had any role in killing Sgt. Speer. Once the youngest detainee at Guantanamo, Mr. Khadr was subjected to sleep deprivation and solitary confinement by his U.S. interrogators.

The Supreme Court of Canada ruled in 2010 that the actions of federal officials who participated in U.S. interrogations of Mr. Khadr had offended "the most basic Canadian standards about the treatment of detained youth suspects."