Charter challenges are piling up against an Ontario government law capping public sector wage increases, with at least 15 unions arguing that it violates workers' collective bargaining rights.

A coalition of 10 unions representing more than 250,000 workers, including the Canadian Union of Public Employees, SEIU Healthcare, and United Steelworkers, announced Tuesday that they are planning a joint constitutional challenge.

The Ontario Nurses' Association also announced Tuesday that it is launching its own court case against the legislation, arguing it infringes on collective bargaining rights.

The announcements came days after the major teachers' unions filed three separate court challenges.

"In terms of challenging this undemocratic legislation, the more the merrier," said Fred Hahn, president of CUPE Ontario."Honestly, we're all united in our opposition against this unfair and unconstitutional bill, and we'll all be working on a political campaign mounted in communities aimed at Tory MPPs to repeal the bill as well."

CUPE President Fred Hahn says the bill to cap wages is unfair and unconstitutional. "We'll all be working on a political campaign mounted in communities aimed at Tory MPPs to repeal the bill as well," he said. (Michelle Siu/Canadian Press)

The law caps all public sector salary increases at one per cent per year for the next three years.

Treasury Board President Peter Bethlenfalvy has said he is confident the law is constitutional, and that the alternative to limiting wage increases is layoffs or tax hikes.

"It supports our ongoing efforts to restore the province to a position of fiscal health and demonstrate respect for taxpayers' dollars," Bethlenfalvy said in a statement.

But the unions say the government should not be balancing its budget on the backs of front-line workers, particularly when it spends $231 million to cancel green energy contracts and voted to increase their own housing allowances by 20 per cent.

'We fought Bill 115'

"No one believes that the [personal support worker] caring for your parents or grandparents in long-term care facilities, or educational assistants or custodians at your children's school, the teaching assistant or food service worker at your university, or the community social service worker in developmental services, or working to protect vulnerable children and families caused the deficit of the province of Ontario," Hahn said.

The unions are buoyed by the fact that CUPE and the teachers won a 2012 court challenge against a law known as Bill 115 that imposed contracts on teachers, froze some of their wages and limited their ability to strike.

The court ruled that the government "substantially interfered with meaningful collective bargaining" and Ontario was left having to pay more than $100 million in remedies to the unions.

"We fought Bill 115 and we were proved to be right," Hahn said.