Locking people up alone in tiny cells for prolonged periods of time, as the Canadian prison system does, has long been called everything from counterproductive to a form of torture.

Now Ontario’s top court has weighed in with more harsh words — and the necessary remedy.

Prolonged solitary confinement “causes foreseeable and expected harm,” which may be permanent and can’t be prevented by monitoring, the Court of Appeal for Ontario ruled last week. As such, it “outrages standards of decency and amounts to cruel and unusual treatment.”

So the court did what the federal government has failed to do — and placed a hard cap on the practice by limiting solitary confinement to no more than 15 days.

And Justice Mary Lou Benotto, who wrote the decision for the panel, went on to say there’s probably good reason to ban the practice entirely for those with a mental illness but the court did not have the necessary evidence to define that in law. That, she wrote, “remains to be determined another day.”

Justin Trudeau’s Liberal government shouldn’t wait. It should take proactive action now.

This ruling already upends the way Ottawa operates prisons and its longstanding and shameful practice of placing prisoners in solitary for weeks, months, and in some truly tragic cases, years at a time.

The government needs to overhaul Bill C-83, its attempt at reform which is now before the Senate. That bill felt short of what was needed before this ruling, and it’s even worse now.

Sadly, that’s not how Public Safety Minister Ralph Goodale sees it. “We continue to work to pass Bill C-83, which will eliminate segregation and establish a fundamentally different system,” his spokesperson said.

The government’s legislation does not eliminate solitary confinement; it simply redefines it.

Administrative segregation, the official term for solitary, is to be replaced with “structured intervention units.” And prisoners would be allowed to spend four hours a day outside their cell and at least two of those hours interacting with other people.

That’s certainly a better version than we have now and, conveniently, it falls just outside the United Nations technical definition of solitary confinement. But the bill doesn’t go far enough.

And crucially, given Thursday’s court ruling, the legislation does not include a limit on how long prisoners can be held in this updated version of isolation.

So, yet again, the court has leapfrogged the government’s too-timid fix and rightly demanded more. That’s why the government shouldn’t wait for the next case to come along to determine that isolation is never the right solution for inmates with mental illnesses.

To its credit, the Trudeau government has taken major steps to reduce the use of solitary and dropped the daily averages from nearly 800 people in 2014 to something closer to 300 this year.

But it’s still overused. The prison system argues that administrative segregation is necessary to maintain security, protect inmates from each other and themselves. But isolation can’t be the simple solution to what are complex problems.

A time limit to the use of isolation is necessary because without it there is no impetus for authorities to find real solutions for these inmates, many of whom suffer from serious mental health issues that are only made worse in solitary confinement.

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After all, there have already been countless reports by prison watchdogs, coroner inquests into tragic deaths by suicide after prolonged stints in solitary, and court rulings calling for dramatic change.

“With this decision the Court of Appeal has brought to an end a sorry chapter in the administration of Canada’s prisons,” said the Canadian Civil Liberties Association, which launched the court challenge.

Let’s hope this time that proves to be true.