New York City Opera’s future darkened this weekend when the company declared a lockout for the first rehearsal of the season, scheduled for Monday, amid a labor dispute with its unions. The impasse raises the possibility that the struggling company may have to cancel its first production, “La Traviata,” and possibly the season.

The move came after mediated talks broke down on Saturday night. Negotiations between the company and its two unions, Local 802 of the American Federation of Musicians, and the American Guild of Musical Artists, have grown increasingly bitter since City Opera announced last spring that it was moving out of its Lincoln Center home, the David H. Koch Theater, to save money.

Local 802 represents the orchestra, and the guild represents the chorus and singers as well as stage managers and directors.

The lockout most immediately affects the chorus, which was scheduled to report for a “Traviata” rehearsal at 3 p.m. on Monday, along with a rehearsal pianist. The orchestra is not scheduled to rehearse until Feb. 1. City Opera’s spokeswoman, Risa B. Heller, said the company had no plans to hire replacement workers.