A man who shot dead six worshippers at a Quebec mosque in 2017 had his sentence reduced to 25 years in prison Thursday when a Canadian court ruled it was unconstitutional for him to serve consecutive life sentences.

Alexandre Bissonnette, who turns 31 next week, was sentenced in 2019 to life in prison with no possibility of parole for 40 years.

In a unanimous decision, the Quebec Court of Appeal said a provision of the Criminal Code introduced in 2011 that allows judges to impose consecutive life sentences for multiple murders violates the Canadian Charter of Rights and Freedoms.

The three-judge panel said it "makes it possible to impose a penalty which will at all times be cruel and unusual, and grossly disproportionate."

The prosecution had asked for a 150-year sentence, which would have been the longest ever in Canada, while the defense petitioned for 25 years.

The trial judge handed Bissonnette 40 years, estimating that "subjecting a murderer to a sentence greater than his life expectancy" risked "sowing doubt as to the credibility of the judicial system."

Bissonnette was 27 at the time of his arrest.

In doing so, Quebec Superior Court Justice Francois Huot had effectively rewritten Canada's sentencing laws, which the appeals court said was also a mistake.

The appeals court said Huot should have "invalidated" the sentencing provision and handed Bissonnette concurrent sentences in accordance to the law as it was written prior to 2011.

But it added this was still a life sentence, and parole is not assured after 25 years.

On January 29, 2017, Bissonnette burst into the Quebec City mosque and unleashed a hail of bullets on the 40 men and four children who were chatting after evening prayers.

He methodically fired dozens of shots, retreating to a safe area to reload his nine-millimetre pistol at least four times, "like he was playing a video game," recounted one witness at his trial.

Six men were killed and five were seriously injured.

The victims were all dual nationals who emigrated to Canada: two Algerians, two Guineans, a Moroccan and a Tunisian.

Bissonnette was described after his arrest as a white supremacist opposed to Muslim immigration but not affiliated with any group.

Quebec's directorate of criminal prosecution said it was reviewing the appeals court decision, which could be taken up by the Supreme Court of Canada.

Boufeldja Benabdallah, co-founder of the Quebec mosque, said he was "disappointed" by the revised sentence, noting that the victims' families must relive the tragedy at each judicial stage.