TORONTO -- TORONTO -- A one-day strike that will close high schools at 10 boards across Ontario on Wednesday is a "virtual certainty," the teachers' union said, with no further contract talks scheduled with the government until the new year.

Harvey Bischof, president of the Ontario Secondary School Teachers' Federation, said Tuesday a government-appointed mediator has ended talks after determining the two sides were too far apart to continue.

The government and union had been scheduled to bargain Monday and Tuesday, but continuing negotiations would be more frustrating than productive, Bischof said.

"Having both sides take some time to consider their positions and return to the table is a reasonable recommendation from the mediator," he said.

With no progress on the horizon, Bischof said Wednesday's planned one-day strike in select school boards is all but certain to go ahead.

The planned one-day strike is the third job action by the union in as many weeks in a bid to ramp up pressure on the government and will affect boards in nearly every region of the province. It follows a provincewide job action earlier this month that shuttered every public high school and some elementary schools, as well as another one-day strike last week that affected nine boards.

The teachers have been without a contract since their last deal with the government expired at the end of August, along with contracts with all public education staff in the province. The government has reached agreements with two unions representing education workers, but remains at odds with a number of other major teachers' unions.

Public high school and elementary teachers are conducting work-to-rule campaigns and say they are pushing back against government plans to increase class sizes and the introduction of mandatory online courses.

Late Tuesday afternoon, Education Minister Stephen Lecce called on the union to cancel the strike.

"This continued disruption only causes uncertainty for parents and their children," Lecce said in a statement. "I remain committed to reaching deals at the table, and call on OSSTF to accept private mediation, and to put forward proposals to advance negotiations."

Lecce has said the main issue in the talks is compensation, with the government recently passing legislation to cap annual wage increases for all public sector workers at one per cent for three years. The union is asking for inflationary increases, which would amount to about two per cent.

The boards affected by the latest planned job action by OSSTF:

Lakehead District School Board

Thunder Bay Catholic District School Board

Lambton Kent District School Board

Thames Valley District School Board

Waterloo Region District School Board

Waterloo Catholic District School Board

York Region District School Board

York Catholic District School Board

Halton District School Board

Kawartha Pine Ridge District School Board

This report by The Canadian Press was first published on Dec. 17, 2019.