Federal authorities are mistreating Indigenous prisoners by failing to ensure that their psychological assessments are not culturally biased, putting the offenders at risk of being unfairly denied parole or rehabilitation programs, the Supreme Court of Canada says.

In a damning critique, the Supreme Court said the Correctional Service of Canada had known since 2000 that several psychological assessments might be unreliable and inaccurate for Indigenous prisoners, but did not do anything to find out if they were. As a result, it failed to live up to what the court called a “guiding principle” — set out by Parliament in a law passed more than 25 years ago — that prison authorities must meet the special needs of Indigenous offenders.

“Two and a half decades have passed since this principle . . . was incorporated into [federal corrections law],” Chief Justice Richard Wagner wrote for a 7-2 majority. “Nonetheless, there is nothing to suggest that the situation has improved in the realm of corrections.”

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He cited reports showing “that the gap between Indigenous and non-Indigenous offenders has continued to widen on nearly every indicator of correctional performance,” mentioning higher security classifications, more time in segregation and being kept behind bars for longer.

Almost 85 per cent of aboriginal inmates are held until their statutory release date (at two-thirds of their sentence), compared with just less than 70 per cent for non-aboriginal inmates, according to a 2015 report from the Public Safety Ministry. Aboriginals make up 42 per cent of the prisoners who are held until the end of their sentence.

The court issued a declaration that the Correctional Service had breached the rights of Indigenous prisoners. If the service wishes to continue using the psychological tests, it must at a minimum conduct research related to bias and Indigenous offenders. Depending on what it finds, it may have to stop using the tests.

The ruling comes in a court challenge brought by a convicted sex-killer, Jeffrey Ewert, who is Métis. Mr. Ewert, 56, has served more than 30 years in prison for second-degree murder and attempted murder, the majority in maximum security. He has not applied for parole, although he became eligible more than 20 years ago, because he says his risk of reoffending as determined in several psychological tests is too high to permit release. He first challenged the tests in an internal prison process in 2000.

The federal government argued he had failed to show the tests were biased against Indigenous prisoners. Mr. Ewert’s position was that the government needed to show the tests were reliable. He won at Federal Court and lost at the Federal Court of Appeal.

The outcome is a bookend to the ruling in a 1999 case called Gladue, in which the Supreme Court directed lower-court judges to give special consideration to aboriginal offenders in sentencing. That case turned on the interpretation of a 1996 law specifying that jail was to be a last resort for all offenders, with particular attention to the circumstances of aboriginals. The Ewert case focused on a 1992 law requiring correctional programs to be responsive to the needs of women, aboriginals, people with mental-health issues and other groups.

“Now, 20 years later, the court is saying to Correctional Services, ‘You really have to do something about this. There is an expectation that this is going to be front and centre in all of your work,’ ” Jonathan Rudin, program director of Aboriginal Legal Services of Toronto, which intervened in the case, said in an interview.

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The correctional service said it is reviewing the decision and “will determine next steps,” a spokesperson said in an e-mail to The Globe and Mail, adding: “It is important to note that culturally appropriate interventions and reintegration support for First Nations, Métis and Inuit offenders is a priority of CSC.”

For Mr. Ewert, the ruling means “more of a fair shake” when he applies for parole, said one of his lawyers, Jason Gratl of Vancouver.

Mr. Gratl said it was hypocritical of the Liberal government, which has made reconciliation with Indigenous peoples a priority, to fight the case. “The Trudeau government has been forced into treating Indigenous inmates equitably,” he said in an interview.

But Scott Bardsley, a spokesman for Public Safety Minister Ralph Goodale, said the government actually won on a key point — that prison authorities had not violated Mr. Ewert’s constitutional rights.

Mr. Bardsley said Ottawa allocated more than $120-million in last year’s budget to support the reintegration of incarcerated Indigenous people and advance restorative justice approaches, and almost $80-million in the previous two budgets to expand mental-health care in correctional facilities.

Mr. Gratl said the ruling could affect more than just Indigenous people — but much depends on the response by prison authorities.

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“How it plays out remains to be seen,” he said. “The Correctional Service of Canada has historically displayed a remarkable resilience in resisting judicial pronouncements.”

In his view, the Supreme Court intended that “all decision-making operations of the Correctional Service of Canada require bureaucratic reflection to determine whether they comport with broad, equity-based considerations.”

The two dissenters, Justice Malcolm Rowe, the first Trudeau appointee to the Supreme Court, and Justice Suzanne Côté, said the prison authorities had lived up to their obligation to address the needs of aboriginal offenders by keeping accurate records of the tests.