The city says about 80 Codiac Transpo drivers, servicemen and mechanics represented by the Amalgamated Transit Union Local 1290 will be in a legal strike position on March 31.
The city says a seven-day cooling off period has begun in the contract dispute because the province did not appoint a conciliation board.
Both sides have agreed to mediation and the city says they will return to negotiations throughout next week.
Outstanding issues in the dispute include wages, scheduling and overtime.
The union has been without a contract since June 2010.