TORONTO -- The union representing Ontario's Catholic school teachers has filed a complaint against the provincial government with the Ontario labour relations board over recent changes to class sizes.

In a statement to members obtained by The Canadian Press, the Ontario English Catholic Teachers' Association said the province essentially pre-empted contract negotiations when it amended regulations related to class sizes last month.

The changes violate provisions in the School Boards Collective Bargaining Act and the Ontario Labour Relations Act that state the terms of a contract agreement cannot be changed while bargaining is underway, the union said.

"We believe this is in violation of the government's obligation to bargain in good faith," the statement said.

The government announced in the spring that high school class sizes would rise from an average of 22 to 28 students over four years. Class sizes for Grades 4 to 8 will increase by one student per classroom, from 23 to 24.

The province has said that will mean 3,475 fewer teachers in the system over four years, a reduction meant to be achieved through not filling vacancies when teachers quit or retire.

Education Minister Stephen Lecce said last month the province is open to negotiating a smaller increase to class sizes.

A spokeswoman for the minister declined to comment on the specifics of the complaint as the matter is before the board, but said the province is focused on reaching an agreement with the union.

"Our government will not be deterred from our mission to reach a deal that ensures that students remain in class. We will continue to bargain with our union partners in good faith, to ensure our kids remain in class," Alexandra Adamo said in an email.

The changes have also been a concern for those in the public school system.

The Canadian Union of Public Employees, which represents education workers, said earlier this week that its members voted overwhelmingly in favour of a potential strike.

The union will be in a legal strike position on Sept. 30 and a mediator has imposed a media blackout on the ongoing bargaining talks.

Contracts for Ontario's public school teachers and education workers expired Aug. 31, and the major unions are in various but mostly early stages of bargaining.