NDP Leader Tom Mulcair made the request directly to Stephen Harper in a letter sent Wednesday to the Prime Minister's Office, a copy of which was obtained by The Canadian Press.
Earlier in the day, Harper told a news conference in Quebec the federal government is deeply concerned with the terrorism verdict and the seven-year sentence handed down to Fahmy and two other Al-Jazeera journalists.
Foreign Affairs Minister John Baird has also said the government will continue to press for his release.
Nonetheless, the government now has the power to revoke Fahmy's Canadian citizenship under its controversial new federal citizenship law, Mulcair noted in the letter.
Bill C-24, which went into effect just last week, gives the government the added discretion to revoke the Canadian citizenship of someone convicted of criminal offences such as treason and terrorism.
Though Fahmy's conviction in Egypt has faced wide international condemnation, Mulcair asked Harper to essentially take the option off the table.
In the letter, Mulcair reminded Harper that "the NDP has firmly expressed our opposition to Bill C-24," noting experts have expressed concerns that it contained "serious violations" of Charter rights.
"Mohamed Fahmy's case in Egypt offers an immediate opportunity to realize the dangers of this new law," Mulcair wrote.
"Are you able to provide assurances that he will not be stripped of his Canadian citizenship when he needs our country's support now more than ever?"
In an email to The Canadian Press, Harper's spokesman Jason MacDonald did just that.
"We have no intention of doing that whatsoever," MacDonald said. "This is another conspiracy theory from the opposition. We have and will continue to advocate for Mr. Fahmy at senior levels."
Earlier in the day, Harper said he has expressed Canada's displeasure to the Egyptian government.
The comments were Harper's first on the issue since Fahmy and his fellow Al-Jazeera journalists, Australian Peter Greste and Egyptian Baher Mohamed, were convicted in a Cairo court Monday and sentenced to seven years in prison.
The three journalists were convicted of giving support to the Muslim Brotherhood, which Egypt labels a terrorist group.
The journalists deny they are terrorists and say they were just doing their job.
Their continuing imprisonment has sparked harsh international condemnation of Egypt.
And it has also opened the Harper government to widespread criticism for what some say is a tepid response, compared to the tough language coming from the United States, Britain and Australia.
"We have been very clear on our concerns — deep concerns — about not just the verdict but about this process from the beginning," Harper said. "We have expressed those to the authorities."
Harper also said Canada has been providing consular support and services to Fahmy from the outset.
Baird said the government is trying to have Fahmy freed on appeal, or through a possible presidential pardon. Though Egypt's president has ruled out intervening in the case, legal experts are divided on whether the imprisoned trio has the possibility of being freed as part of the ongoing legal process.
"Obviously there are some difficult circumstances here," Harper said. "But the Egyptian authorities are very aware of the position of the government of Canada, and we will continue to press that position going forward."
In an interview, Mulcair said he saw in Baird's comments "no real intention to do anything, paying lip service to our international obligations."
Baird has that said Fahmy's case is complicated by the fact that he is a dual Egyptian and Canadian national and also by the nature of the relationship between Al-Jazeera, its Qatari ownership and Egypt's banned Muslim Brotherhood.
The government of Qatar provides financial support to the Muslim Brotherhood, but no one seriously believes that Fahmy was in "cahoots" with the organization, Baird added.
Egypt's former top soldier, Abdel-Fattah El-Sissi, was elected president last month, completing a transition to power that started last July when millions of Egyptians took to the streets to demonstrate against Islamist President Mohammed Morsi.
The Muslim Brotherhood has since been rebranded a terrorist group. The government accused Fahmy and his two colleagues of compromising Egypt's national security though its reporting on the Brotherhood.
The case raises serious questions about media freedom, and it has been widely denounced, including by the international rights' watchdogs Amnesty International and Human Rights Watch.
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