A union representing Ontario's public high school teachers has walked away from contract talks with the provincial government.

The union announced the Ontario Secondary School Teachers' Federation has suspended talks in hopes that the Ontario Public School Boards' Association will reconsider its position.

On Twitter, the OSSTF wrote that negotiators "will return when (the) employer gets serious."

In a bargaining update sent confidentially to CTV Toronto, the union said the OPSBA and government team refuses to remove some sections from the bargaining contract.

"Management's position ... constitutes nothing less than a narrowly-focused management rights agenda designed to erode the working conditions of OSSTF/FEESO members and the learning conditions of Ontario students," the bargaining update said. "At this point, we have reached an impasse and have suspended our participation in central table negotiations."

Earlier this month, the union threatened to withdraw services at seven school boards (Peel, Durham, Halton, Ottawa-Carleton, Waterloo, Rainbow and Lakehead) if a deal isn't reached.

The union told CTV Toronto the Durham District School Board would be the first to launch a strike, and teachers could walk off the job as early as April 20.

The Peel District School Board sent a note to parents warning that the earliest teachers in that board could be on strike is April 25, as the union is required to give five days' notice. In event of a strike, all schools will be closed to students, extracurricular activities and trips will be cancelled and buses will not operate. Teachers may picket outside of Peel secondary schools.

"Our priority during any job action is the safety of our students. We will continue to monitor the situation and its impact on Peel board students," the statement said.

When asked about the development, Environment Minister Liz Sandals said she wasn't taking the move lightly, but said she didn't think parents should see Wednesday's events as an end.

"I'm not going to get into individual issues, but it's the nature of negotiations," she said. Sandals said some new issues had emerged, but did not provide specifics.

"You're dealing with one set of issues and you make some progress, but you turn to a different set of issues and you make less progress."

The negotiations are taking place as the province tries to eliminate a $10.9-billion deficit, with a 2018 deadline. Though the budget will not be revealed until next week, the province has told public servants, teachers and other unions that any wage increases will have to be offset to achieve "net zero."

Union members say the wage freeze is not the main issue driving strike threats, but rather teachers want improvements to working conditions and are looking for more time with students.

With files from CTV Toronto's Naomi Parness and The Canadian Press