Four of Ontario's biggest education unions are launching court challenges to legislation introduced by the province's Progressive Conservative government that caps public sector wage increases.

At a news conference Thursday morning at Queen's Park, the heads of the four unions said that Bill 124 "seeks to unilaterally impose limits on the ability of workers to negotiate compensation" and infringes on charter-protected rights to collective bargaining.

The controversial Protecting a Sustainable Public Sector for Future Generations Act, passed this fall, caps public sector wage hikes at an average of one per cent annually for the next three years.

It applies to colleges, universities, school boards, hospitals, the public service and children's aid societies, among other bodies.

The provincial government passed the law as contract talks were just starting, and the unions say it was an extraordinary interference in the bargaining process.

The law amounts to a "clear demonstration of bad faith on the part of the government," said Sam Hammond, president of the Elementary Teachers' Federation of Ontario.

"By attempting to legislate unilateral limits on compensation growth, the government is turning its back on the collective bargaining process," he continued.

When it was introduced in June, various public sector unions vocally condemned the bill.

In a November statement, a coalition of unions said the bill "tramples on collective bargaining rights and targets public sector workers with unfair austerity measures for the next three years."

"The legislation ensures that compensation for educators and other public sector workers will continue to fall behind the rate of inflation," the statement said.

The following education unions are launching challenges:

Elementary Teachers' Federation of Ontario.

Ontario Secondary School Teachers' Federation.

Ontario English Catholic Teachers' Association.

Association des enseignantes et des enseignants franco-ontariens.

The paperwork will be filed Thursday afternoon, the union leaders said.

This latest development comes amid job actions by both elementary and secondary school teachers.

Public high school teachers at nine Ontario boards held their second one-day strike on Wednesday, with their union blaming the provincial government for failure to reach a resolution in contract talks.

Meanwhile, public elementary school teachers recently intensified their work-to-rule campaign.

Ontario Education Minister Stephen Lecce has repeatedly said that disagreements over compensation have been the main sticking point during negotiations, a charge the unions have denied. Increased class sizes in high schools, mandatory e-learning and job losses among education support workers are also contentious issues at the bargaining table, they said.

Speaking to reporters later Thursday, Treasury Board President Peter Bethlenfalvy defended Bill 124, saying it is constitutional and he wouldn't have proposed it if he didn't think it would stand up in court. He noted that it still allows for employees to get raises for seniority, performance or increased qualifications.

"We do think it's consistent with the charter, we think it doesn't impede the collective bargaining process," he said.

Bethlenfalvy added that the government will review the unions' filings and defend the legality of the bill in court.