The man responsible for killing six people in a Quebec City mosque last year is facing a 150-year prison sentence, which legal experts say would be the longest ever in the country's history.

Alexandre Bissonnette, 28, pleaded guilty to six counts of first-degree murder on Wednesday. Each carries an automatic life sentence, with no eligibility of parole for 25 years.

Quebec Superior Court Justice François Huot may decide to make the periods of ineligibility for parole back-to-back, effectively removing the possibility of Bissonnette ever walking free.

"[Huot] is facing the prospect of imposing the heaviest sentence ever in Canadian judicial history," said criminal defence attorney Walid Hijazi.

Bissonnette entered the Quebec Cultural Islamic Centre on Jan. 29, 2017, shooting at dozens of worshippers minutes after evening prayer ended. Along with the six men he killed, five others were critically injured.

Alexandre Bissonnette's defence lawyer, Charles-Olivier Gosselin, said he will present constitutional arguments at sentencing to avoid consecutive life sentences for his client. (Julia Page/CBC)

After entering his guilty plea on Wednesday, Bissonnette read a statement to the court that expressed regret for his actions.

Bissonnette's lawyers will likely use the statement to ask for more lenient parole eligibility, Hijazi said. They could argue the 28-year-old showed remorse and tried to spare the families the anguish of going through a trial.

"Remorse expressed by an accused is usually considered by a sentencing judge," he said.

Is the sentence constitutional? On Wednesday, the legal aid defence lawyer assigned to Bissonnette, Charles-Olivier Gosselin, said he intended to present evidence detailing his client's mental health and his risk of reoffending. "There will be an extensive case presented to Justice Huot, and he will be able to consider constitutional arguments," Gosselin told reporters. The Criminal Code was amended by Stephen Harper's Conservative government in 2011, to allow a judge to impose consecutive rather than concurrent periods of parole eligibility for multiple murders. In Bissonnette's case, the six terms would add up to 150 years with no chance of parole — 25 for each count of first-degree murder. Several other judges have availed themselves of the option when faced with someone convicted of multiple murders. Justin Bourque was sentenced to life in prison with no chance of parole for 75 years after he pleaded guilty to the murders of three RCMP officers in New Brunswick in 2014. He also admitted to two counts of attempted murder. It was once considered the harshest sentence in Canada since the abolition of the death penalty in 1976.

Boufeldja Benabdallah, president and co-founder of the Quebec Islamic Cultural Centre, said he hopes the judge will set an example with his sentence. (Julia Page/CBC)

But some within the legal community question whether stacking the parole ineligibility is constitutional.

Douglas Garland, who was found guilty in Alberta of three counts of first-degree murder last year, was also given consecutive sentences. He won't be eligible for parole before 2093.

His lawyers appealed his sentence in March 2017, arguing it was "excessive and harsh," and violated his charter rights.

Triple murderer Derek Saretzky is also appealing the consecutive sentences he was given. Otherwise, he will be 97 before becoming eligible for parole.

Bissonnette's lawyers will likely draw on similar arguments to plead against consecutive sentences, said criminal defence lawyer Patrick Davis.

"It's a sentence that could be incompatible with human dignity and disproportionate, to the point where Canadians would consider this sentence as intolerable and odious," Davis said.

Davis compared someone spending their entire life in prison, knowing they will never get out, to bringing back capital punishment. "Sending someone to 150 years in prison, for me, is appaling."

He added the principles of Canada's justice system are based on rehabilitation, and he warned against "Americanizing sentences in Canada."

Hopes for a long sentence

The Crown has not yet disclosed if it will seek consecutive parole ineligibility for Bissonnette.

Prosecutor Thomas Jacques said he wouldn't disclose any information before sentencing hearings for Bissonnette, on April 10, "out of respect for the judicial process."

Douglas Garland was found guilty in the deaths of Nathan O'Brien, 5, and his grandparents, Alvin and Kathy Liknes, in 2017. (Jeff McIntosh/Canadian Press)

"We will limit ourselves to say the [Justice] Ministry will demand a sentence that reflects the magnitude of the crime committed," Jacques said earlier this week.

For his part, the president of the Quebec City mosque where the tragedy unfolded, Boufeldja Benabdallah, hopes the sentence will set an example.

''Society shouldn't have to live through something like this just because someone acts out on a whim, or in an organized attack," Benabdallah said outside the courtroom on Wednesday.

"[The sentence] has to be exemplary so this never happens in Canada again."