TORONTO -- A $125-million class-action lawsuit has been filed against the provincial government for the mistreatment of children in youth justice facilities.

The suit filed with the Ontario Superior Court of Justice alleges the Crown breached its fiduciary duty to ensure the welfare of children by allowing the use of solitary confinement in the province's 20 detention centres for young offenders.

Lawyers say the plaintiff, who cannot be named due to the individual's status as a minor at the time of the offence, was subjected to lengthy periods of solitary confinement at the Genest Detention Centre for Youth in London, Ont.

The allegations, which have not been proven in court, claim "the extensive and improper use of cruel, inhuman and degrading punishments" including extended periods of solitary confinement cause irreparable harm.

Ontario's children's advocate Irwin Elman called on the government in August to stop putting young offenders in solitary confinement for more than 24 hours, warning it negatively harms their mental and physical health.

Lawyer Jay Strosberg says Human Rights Watch, the World Health Organization and the United Nations have issued strong condemnation of the use of solitary confinement on juveniles.

Strosberg says Canadians would be appalled to know the degree to which solitary confinement is used in Ontario's Youth Justice Facilities.

"There are no circumstances where such punishments are reasonable, and the province's inaction is disturbing," he said.

A spokeswoman for the minister of children and youth services said in a statement that Ontario's youth justice system has undergone a "massive transformation" over the past decade and the ministry will continue to work "with the best interest of the youth it serves in mind."