A proposed punishment of life without parole for especially brutal murders, police killings and murders during sex assaults would likely be struck down by judges unless the government softens it, a former legal adviser to Prime Minister Stephen Harper says.

Benjamin Perrin, a constitutional specialist who was lead justice adviser in the Prime Minister's Office in 2012-13, says the punishment should not be mandatory, as proposed, but should be left to judges' discretion on the non-binding recommendation of a jury, he writes in a report to be released on Thursday by the Macdonald-Laurier Institute, a conservative think tank.

Judges should also be able to set parole eligibility at anywhere from 25 to 75 years rather than being limited to a stark choice between 25-year eligibility and no chance at all. Without those changes, and some others, judges would probably rule that the law is unconstitutional, he said, which would continue the government's losing streak in major cases at the Supreme Court.

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"The idea that the courts are soft on crime and we need to make this mandatory is wrong," Prof. Perrin, who teaches at the University of British Columbia's Peter A. Allard School of Law, said in an interview.

As proof that the courts are not too lenient, his report cites the results of a 2011 Conservative law that empowered judges in cases of multiple murder to add the parole eligibility periods together. Judges have done so four times, producing sentences ranging from 30 years (two second-degree murders in a shootout at Toronto's Eaton Centre) to 75 years (first-degree murder of three RCMP officers in New Brunswick).

First-degree murder currently brings a mandatory penalty of life, with parole eligibility beginning at 25 years. Second-degree murder also has a mandatory life penalty, and judges set eligibility at anywhere from 10 to 25 years.

The government's Life Means Life Act, which needs to be debated and approved by Parliament before it becomes law, would make life without parole mandatory for especially "brutal" murders, the murders of police officers and murders during sexual assaults or hijackings. It would make life without parole discretionary for second-degree murder by a person who has committed a previous murder, and in all first-degree cases where life without parole is not mandatory. Cabinet could be asked to grant release after 35 years.

Removing judges' discretion has been a key feature of Conservative crime laws, including dozens of mandatory minimum sentences for everything from drugs to crimes against children to non-violent offences. But the government has often run afoul of the Supreme Court, as Mr. Perrin documented in a report last year for the Macdonald-Laurier Institute.

He said he is not sure why the government keeps passing laws that are declared unconstitutional. "Is it getting bad advice or is it getting good advice that it chooses not to follow?"

Mr. Perrin said he supports making penalties tougher for heinous murders. "Not all murders are created equal. Some are much more deplorable than others," he said. While critics of the bill say the Parole Board of Canada has a good record in deciding which killers are safe to release, Mr. Perrin's report cites board statistics that show 22 per cent of 1,886 convicted murderers granted full parole between 1994 and 2008 were returned to jail. Three per cent committed a violent offence, 6 per cent committed a non-violent offence, and 13 per cent breached parole conditions.

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Clarissa Lamb, a spokesperson for Justice Minister Peter MacKay, said department lawyers review all government bills for compliance with the Charter of Rights before they are introduced. She said the 35-year review by cabinet is intended to meet constitutional concerns.

The bill as written would "ensure that offenders who are convicted of the most heinous crimes or those who are convicted of high treason will be imprisoned for the rest of their natural lives with no access to parole," she said.