OTTAWA -- The federal prison service has failed to ensure its psychological assessment tools are fair to Indigenous inmates, the Supreme Court of Canada says in a ruling that could open the door to a wholesale examination of the techniques.

In a 7-2 decision Wednesday, the high court accepted prisoner Jeffrey Ewert's challenge of five assessment tools the Correctional Service of Canada uses to gauge the risk of reoffending and potential for violence.

It effectively means the Correctional Service must review the tools to make certain they are free of cultural bias, or stop using them altogether.

"For the correctional system, like the criminal justice system as a whole, to operate fairly and effectively, those administering it must abandon the assumption that all offenders can be treated fairly by being treated the same way," a majority of the court said in its reasons.

The decision comes as the Liberal government grapples with the over-representation of Indigenous people in the federal prison system.

In 2015-16, Indigenous offenders represented almost one-quarter of the total federal offender population. A government report also noted a lower percentage of Indigenous offenders benefited from gradual release from custody than non-Indigenous ones.

Chief Bob Chamberlin, vice-president of the Union of B.C. Indian Chiefs, welcomed the court ruling. "Today's decision is a step forward in the fight to reduce the over-incarceration of our people."

The Correctional Service is reviewing the decision and "will determine next steps," said prison service spokeswoman Stephanie Stevenson.

Ewert, who identifies as Metis, alleged the prison service's assessment techniques were not proven to be reliable for Indigenous inmates because they were developed and tested on predominantly non-Indigenous subjects.

He claimed that reliance on the tools violated the Corrections and Conditional Release Act, which requires the prison service to "take all reasonable steps to ensure that any information about an offender that it uses is as accurate, up to date and complete as possible."

Ewert, 56, also contended that use of the tools violated constitutional guarantees of equality and liberty.

Born to a Metis mother and a British father, Ewert was adopted as a baby by a Caucasian family in Surrey, B.C.

Court documents describe his adoptive father as an alcoholic, and his adoptive mother as psychologically unstable and abusive. Ewert was subjected to racism and discrimination both at home and at school.

He has been locked up for more than 30 years in maximum- and medium-security institutions, serving two concurrent life sentences for second-degree murder, attempted murder and escape from custody.

Ewert became eligible for day parole in 1996 and full parole three years later. However, he has waived his right to each parole hearing.

A Federal Court judge found the prison service had breached the corrections act and infringed Ewert's charter right to liberty, but the decision was later overturned -- prompting his appeal to the Supreme Court.

In its decision, the Supreme Court said the Correctional Service must base its decisions about inmates on sound information if it is to ensure the safety of other prisoners, staff members and the general public.

The high court stopped short of concluding Ewert's charter rights had been breached.

However, the court found the Correctional Service "fell short of what it is required to do" by "disregarding the possibility that these tools are systematically disadvantaging Indigenous offenders and by failing to take any action to ensure that they generate accurate information."

The court said that if the prison service wishes to continue using the tools, it must conduct research into whether -- and to what extent -- they apply differently to Indigenous offenders.

Depending on the outcome, the service may need to modify the tools or stop using them on Indigenous inmates, the ruling said.