The federal government is reintroducing a contentious bill that would restrict the release of "high-risk" mentally ill offenders despite opposition from lawyers and mental-health professionals.

The legislation would alter the rules for those found not criminally responsible (NCR) for a crime, in part by giving judges new powers to designate some violent offenders as "high-risk." The bill was studied by a committee last spring but did not become law before Parliament was prorogued.

Justice Minister Peter MacKay said the legislation is needed because of public concern about several high-profile cases, including that of Vince Li, who was found not criminally responsible in 2009 for beheading passenger Tim McLean on a Greyhound bus. Mr. Li was granted permission last spring to leave the mental-health centre where he is being held on escorted day trips.

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"Victims and concerned citizens as well as provinces and territories have raised concerns about NCR accused persons being released too early back into the public and potentially committing further acts of violence," Mr. MacKay said Monday.

Under Canadian law, a court can find individuals not criminally responsible for a crime if they lacked the capacity to understand their own actions. They are usually detained and treated in a hospital until a review board determines that they no longer pose a threat to the public.

Critics of the proposed "high-risk" designation say there is no evidence to suggest the current system isn't working, pointing to significantly lower rates of reoffending among those in the not-criminally-responsible system. They also worry the bill could result in more people with serious mental illnesses ending up in prisons rather than hospitals.

The "high-risk" designation would prevent an individual from being considered for release until a judge decides the designation no longer applies. As a result, some lawyers say the proposed changes could lead people to opt for a criminal trial – and a fixed prison sentence – instead of the not criminally responsible defence.

Catherine Latimer, executive director of the John Howard Society, said there are already significant problems with accessing mental-health programming in federal prisons.

"Most people will have some form of mental-health issues that they're going to be required to deal with," she said. "But the more serious mental-health issues – the capacity of the prison system to deal with those effectively is limited. And even the corrections authorities will say, you know, they'd prefer that those with serious mental-health issues be dealt with elsewhere."

Mental-health organizations have also expressed concern that the bill could stigmatize people with mental-health problems. Chris Summerville, chief executive officer of the Schizophrenia Society of Canada, said his organization and others met with staff from Mr. MacKay's office about the bill recently and were encouraged to suggest amendments when it goes before a Senate committee for review.

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"If we wanted to prevent these high-profile crimes by Canadians with a mental illness, the best tool would be early identification, early intervention and early treatment," Mr. Summerville said. "But this is not a health bill, it's a justice bill."

Mr. MacKay said a high-risk designation would not affect offenders' access to mental-health treatment. He added that the bill contains two amendments from its original version, both of which were proposed at committee earlier this year. One requires the government to conduct a review of the bill five years after it is passed, and a second would ensure victims are told where someone found not criminally responsible intends to live after being released.

"Our government intends to strike a better balance between the need to protect society against those who pose a significant threat to the public and the need to treat mentally disordered accused persons appropriately and proportionately," Mr. MacKay said.