The two men are on the 13th day of a hunger strike in Tora prison, south of Cairo.
Justin Podur, who has been posting updates on his blog, told CBC News he had received information from Greyson and Loubani's lawyers in Cairo about the extension.
Cecilia Greyson, John's sister, said their lawyers told her that everyone arrested on Aug. 16 during the violent protests have had their imprisonment extended for the same amount of time.
Greyson said lawyers have launched an appeal to free the men, with a decision possible within days.
The news comes a day after Podur posted what Greyson and Loubani say is their full account of the day of their arrest.
- FULL STATEMENT: John and Tarek in their own words
- Egypt-held Canadians plan hunger strike
Loubani, an emergency room doctor from London, Ont., and Greyson, a Toronto filmmaker and university professor, had been intending to make their way to the Gaza Strip last month when they were arrested. They both have family in the Maritimes — Greyson's sister lives in Halifax and Loubani has family in both Halifax and Bathurst, N.B.
In their statement, the two say they were stuck in Cairo and had decided to visit a protest in Ramses Square, five blocks from their hotel.
Within moments of arriving at the demonstration, they say chaos ensued, with wounded people being carried out of the square and gunshots ringing out. According to the statement, Loubani began helping the injured, at one point performing CPR, while Greyson filmed the scene. They say they witnessed the deaths of at least 50 people.
'Slapped, beaten, ridiculed'
Afterward, the pair say they were having trouble getting out of the police cordon and back to their hotel, asking for help at a checkpoint.
“That's when we were: arrested, searched, caged, questioned, interrogated, videotaped with a 'Syrian terrorist', slapped, beaten, ridiculed, hot-boxed, refused phone calls, stripped, shaved bald, accused of being foreign mercenaries. Was it our Canadian passports, or the footage of Tarek performing CPR, or our ice cream wrappers that set them off? They screamed 'Canadian' as they kicked and hit us. John had a precisely etched bootprint bruise on his back for a week.”
According to Tarek’s brother, Mohammed, Canadian consular staff documented the pair’s injuries but told the families of the two not to publicly reveal the two men had been beaten.
“They were worried that the Egyptian authorities might cut our access off to Tarek and John if we publicized their mistreatment," he told CBC News.
Mohammed is urging the federal government to take the issue to a higher level.
"We believe it is now past due for the Prime Minister to call the Egyptian Prime Minister directly and highlight how damaging to Canada-Egypt relations this whole episode has been," he said in an email.
Officials at the Department of Foreign Affairs weren't immediately available for comment about Loubani and Greyson's alleged mistreatment, or news of their continuing detention.
CBC News has reached out to the Department of Foreign Affairs for comment about the latest events, but there has been no answer yet.
Béatrice Fénelon from Foreign Affairs issued an official statement Saturday that "Canada continues to press at all levels, including directly with the Egyptian Foreign Minister, for a timely and positive resolution to this situation and in the absence of confirmation of the charges continues to call for the release of Dr. Loubani and Mr. Greyson."