The Supreme Court of Canada ruled Friday that a provision in Canada's Youth Criminal Justice Act is unconstitutional for forcing young offenders to prove why they should not be punished as adults for serious crimes.

'[The provision] clearly deprives young people of the benefit of the presumption of diminished moral blameworthiness based on age.'— Supreme Court ruling

In a 5-4 ruling written by Justice Rosalie Abella, the court deemed the reverse onus provision of the act is in violation of Section 7 of the Canadian Charter of Rights and Freedoms.

The reverse onus provision places responsibility on teenagers convicted of serious or violent crimes, including murder, manslaughter and aggravated sexual assault, to prove to the courts why they should be sentenced as juveniles instead of adults. It was introduced by the former Liberal government in 2002 and was a major challenge to a nearly century-old approach that saw teenagers treated more leniently than adults.

"[The provision] clearly deprives young people of the benefit of the presumption of diminished moral blameworthiness based on age," the ruling read.

"By depriving them of this presumption because of the crime and despite their age, and by putting the onus on them to prove that they remain entitled to the procedural and substantive protections to which their age entitles them, including a youth sentence, the onus provisions infringe a principle of fundamental justice."

The provision has already been deemed unconstitutional by appeal courts in Quebec and Ontario, although British Columbia's appellate court has upheld it.

Ruling could affect many cases across country

Reporting from Ottawa, the CBC's Julie Van Dusen said the ruling doesn't mean an adult sentence cannot be imposed on a young person, but that it's now up to the Crown to justify why an adult sentence is warranted.

"This is a very far-reaching decision by the courts today," Van Dusen said.

"And one that certainly involves many, many cases throughout the country and one that will be looked at carefully by the government."

Friday's ruling stems from a case in 2003 involving two teenagers, aged 17 and 18. The 17-year-old, known as D.B., was convicted of manslaughter after the other teen died following a fight outside a Hamilton shopping mall. While D.B. received a three-year intensive rehabilitative custody and supervision order, prosecutors opposed the youth sentence and sought an adult term of five years in prison.

The ruling addresses one of the founding principles of the juvenile justice system, which in Canada was hived off from the adult system in 1908 with the creation of the Juvenile Delinquents Act, which eventually became the Youth Criminal Justice Act in 2003: does a young offender deserve to be treated differently, "not as a criminal, but as a misdirected and misguided child?"

The Supreme Court's decision on Friday affirms the idea that teens, particularly those aged 14 to 17, should be presumed less morally culpable for their crimes than adults and therefore have a constitutional right to be treated differently.

Harper's government watching case closely

Four of the court's nine justices, however, gave a dissenting opinion. They said that, when it comes to violent young offenders, it is "entirely appropriate" for Parliament to consider the competing interests of public safety and the reduced moral blameworthiness of youth.

As part of its crime platform, Stephen Harper's Conservative government has said it wants young people who commit serious crimes to be treated as adults and receive stiffer sentences, a move the Conservatives say would deter would-be youth criminals and cut down on repeat offenders.

Harper's government is slated to review the Youth Criminal Justice Act sometime in 2008 and has suggested the act could be in for an overhaul. Friday's court ruling, however, may not bode well for the Conservatives' proposed crime platform.

"This is the latest in a string of decisions made by the Supreme Court where they have not liked the idea of reverse onus, so it foreshadows some trouble ahead," the CBC's Alison Crawford reported from Ottawa.

The government has passed a number of laws that feature reverse onus provisions for people such as dangerous offenders or those who have committed serious gun crimes and are seeking bail.

Crawford said the government may also take heat from the opposition, who were present at committee meetings when expert witnesses warned that such laws may not withstand Supreme Court challenges based on rights guaranteed in the Charter of Rights and Freedoms and the Constitution.

With files from the Canadian Press