Lawyers in Ontario are calling for an end of solitary confinement in provincial youth-justice centres, a practice they say is cruel punishment that can cause mental health issues.

A $125 million class-action lawsuit calling for the end of the practice was filed on Wednesday.

"It's incredibly harmful when you place a young person, a child, into solitary confinement," Toronto lawyer James Sayce told CTV News Channel on Thursday. "Time passes in a different way for children. It can create or exacerbate certain mental health issues that many of these children have."

Sayce, who's involved in the lawsuit, said the World Health Organization and the United Nations have said the use of solitary confinement for juveniles can constitute cruel and degrading punishment, and in some cases it could be considered torture.

In Ontario, young people under the age of 16 cannot be kept in solitary confinement for more than eight hours in any one day, or 24 hours in any one week. Those aged 16 and older cannot be held for more than 72 hours.

A report released by the Provincial Advocate for Children and Youth in August said the number of young people held beyond 24 and 72 hours is "concerning."

"There are indications that serious mental health concerns emerge or are exacerbated during the use of secure isolation in Ontario," the report reads.

The lawsuit alleges that children as young as 12 are being place in isolation.

"You shouldn't be putting 12-year-olds in solitary confinement," Sayce said, "It's never OK."