The man who murdered six men in a Quebec City mosque in January 2017 is facing the prospect of being handed the most severe prison sentence ever imposed in Canada.

On Monday, lawyers began debating the fate of Alexandre Bissonnette.

Bissonnette, 28, pleaded guilty earlier this year to six charges of first-degree murder and six of attempted murder.

While his first-degree murder conviction carries an automatic life sentence with no chance of parole before 25 years, Bissonnette can also receive consecutive terms, which means he could be sentenced to up to 150 years.

The judge in the case has said he is not ruling out a sentence longer than 25 years.

But before deciding on Bissonnette's fate, Quebec Superior Court Justice Francois Huot will have to rule on a defence motion arguing consecutive sentences — a part of the Criminal Code since 2011 — are unconstitutional and invalid.

Bissonnette's lawyers began making their case Monday, arguing cumulative sentences amount to a "death sentence by incarceration."

Defence lawyer Charles-Olivier Gosselin did not call any witnesses Monday but presented the judge with numerous documents, including academic studies regarding life expectancies, deaths and suicides for people in detention.

Huot has set aside several court days to hear arguments on the defence's motion as well as Bissonnette's sentence. Huot has previously warned he doesn't expect to make a ruling on Bissonnette's fate before September.

The widow of one the victims had told the court how a lighter sentence or the prospect that Bissonnette would be released would amount to a second death for the victims of the attack.

Crown prosecutor Thomas Jacques has promised to seek a sentence that will reflect the extent of the "heinous crimes" committed.

The Attorney General of Quebec has stated that a cumulative sentence for multiple murders is "constitutionally valid."

In a 54-page document filed last week in Quebec Superior Court, the Crown invited Huot to reject the constitutional challenge initiated by Bissonnette's lawyers.

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