The Ontario English Catholic Teachers' Association, which represents 50,000 teachers, says it is taking the action after talks involving a conciliator failed to lead to an agreement with the provincial government and trustees.
In a confidential update sent to members on Thursday and obtained by The Canadian Press, the union says it will be in a legal strike position on Aug. 17 and strike action will begin on the first scheduled instructional day of the 2015-16 school year.
The update says strike action will initially take the form of an enhanced work-to-rule and specific details will be provided to members on Aug. 11.
Its members voted 94 per cent for a strike mandate in April.
Education Minister Liz Sandals said in a statement Thursday afternoon that the government is prepared to compromise and work through issues to reach an agreement with the teachers.
"We want students and teachers to start the new school year without disruptions," Sandals said.
Several major teachers' unions have raised the possibility of September strikes, listing class sizes, control over teacher preparation time and hiring practices as sticking points in negotiations.
"The government's aim is to make profound cuts to our collective agreements that would have a negative impact on students, their families, and our hard working teachers, who are already stretched to the limit," Ann Hawkins, OECTA president, said in a statement Thursday.
More than 70,000 public high school students were out of school for weeks this spring due to strikes in the Toronto and Sudbury areas.
The Ontario Labour Relations Board eventually ruled that the teachers' strikes in the Durham and Peel regions and the Rainbow District were illegal.
In an attempt to avoid further strikes in those boards in that school year, the Liberal government enacted back-to-work legislation.
Elementary teachers have been on an administrative strike since May.