The high court will deliver its ruling in a pair of cases involving mandatory minimum sentences for gun crimes.
The Ontario Court of Appeal struck down the law in 2013, calling it "cruel and unusual punishment," so the provincial attorney general and its federal counterpart are appealing the ruling.
The Appeal Court struck down both the three-year mandatory minimum for a first offence of possessing a loaded prohibited gun, as well as the five-year sentence for a second offence.
The Ontario and federal governments are urging the Supreme Court to reverse that ruling, arguing the law does not breach the charter protection against cruel and unusual punishment.
The new sentencing rules were enacted in 2008 as part of a sweeping omnibus bill introduced by the federal Conservatives.
The Ontario ruling is already binding in the province, and can be taken into consideration by judges in other provinces in similar cases.
Tuesday's ruling will settle the matter nationally.
The high court has agreed to hear two cases, one on each of the two sentencing provisions.
In one case, a young Toronto man with no criminal record was sentenced to three years in prison after pleading guilty to possession of a loaded firearm.
The judge said that had there not been a law calling for a mandatory minimum, he would have sentenced Hussein Nur to 2 1/2 years.
"Parliament's increase of the mandatory minimum from one year to three years was a forceful response to the dangers posed by the proliferation of handgun possession and a determination to combat this 'serious and growing societal danger,'" Ontario and the federal government argue in their factum.
"It was open to Parliament to create an aggravated form of firearm possession with enhanced punishment."
In the second case, Sidney Charles pleaded guilty to firearms offences after he was found in his rooming house bedroom with a loaded and unlicensed semi-automatic handgun. He was sentenced to five years because he had two previous convictions.
In defending the mandatory sentence for repeat offenders, Ottawa and Ontario argue that it is not overly punitive.
"The overall effect of raising some offenders' sentences to five years is within a reasonable range of legislative choice, particularly given the importance of denouncing and deterring this dangerous conduct which puts the public at risk," they argue in their factum.
After several cases in which it clashed with the Conservatives in several key policy areas, the Supreme Court recently sided with Ottawa in its recent ruling that said it had the right to legislate the destruction of Quebec's gun registry data.
The high court upheld the notion that Parliament has the power to make criminal law under the Constitution, as long as it operates within the law.
That win for the Conservatives came after several losses at the Supreme Court. They included rejecting its appointment of Quebec judge Marc Nadon to its ranks, rejecting Parliament's right to reform the Senate on its own and upholding the right of Vancouver's controversial Insite safe-injection facility to stay open when the government wanted it shut down.
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