By Shadi Bushra

CAIRO (Reuters) - A former aide to ousted Egyptian president Mohamed Mursi of the Muslim Brotherhood was released on Sunday for health reasons after 18 months in detention without charge, relatives said.

Khaled al-Qazzaz, Mursi's former foreign affairs secretary, was arrested in July 2013 along with the elected president and eight other senior aides when the army removed the Brotherhood from power following mass protests against its rule.

Egyptian officials were not immediately available for comment on Qazzaz, one of the only Brotherhood-linked figures to be released from jail since security forces arrested thousands of supporters of the Islamist movement.

"The guards at the door to his room came to him a couple of hours ago, and told him 'we've been ordered to leave and you're free to go,'" Ahmed Attia, his brother-in-law told Reuters by telephone from Canada.

Qazzaz is staying at his parents' home in Cairo and hopes to rejoin his wife and four daughters in Toronto soon, Attia said.

A public prosecutor told reporters on Sunday that Qazzaz was being released on health grounds. He said Qazzaz had been under investigation for inciting violence and joining a terrorist group, a reference to the Brotherhood, which says it is a peaceful movement.

It is still not clear if Qazzaz, an Egyptian citizen, will be allowed to leave the country for Canada, where he holds permanent residence.

Relatives said he was a member of the Freedom and Justice Party, the political wing of the Brotherhood, not a card-carrying member of the group.

They said he was held under guard at a hospital for the last two months after more than 400 days in solitary confinement.

A U.N. panel said last year that he was held unlawfully.

Qazzaz's relatives declined to comment on whether there may have been political motives behind his release.

"I'm not a politician, I'm just a wife and a mother and I'm just happy my husband is released and I hope we can be reunited soon," Qazzaz's Canadian wife, Sarah Attia, said by telephone.

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"My husband had been detained for 558 days now, and he's never been charged."

In a separate case, speculation has grown that Egypt will release three journalists from the Qatar-based Al Jazeera station who have been in prison for about a year.

Australian Peter Greste, Canadian-Egyptian Mohamed Fahmy and Egyptian national Baher Mohamed were sentenced to seven to 10 years on charges including spreading lies to help a "terrorist organization" -- a reference to the outlawed Brotherhood.

They deny the charges. Al Jazeera has called the accusations against its journalists absurd.

Ties between pro-Brotherhood Qatar and Egypt were strained after the army toppled Mursi. But Saudi-brokered efforts to repair ties have raised the possibility that the journalists will be freed.

(Additional Reporting by David Ljunggren in Ottawa and Stephanie Nebehay in Geneva; Editing by Michael Georgy and Stephen Powell)