Plans for a fall season at the Metropolitan Opera suffered another setback yesterday as the last of the "big three" unions which has still not settled its labor dispute voted to reject what the Met said was its "final" contact offer. Shortly afterward, the director of the Federal Mediation and Conciliation Service Wayne L. Horvitz, called both sides to a new bargaining session scheduled for 1 p.m. today.

The American Guild of Musical Artists -- which represents about 350 full and part-time solo singers, superstar singers, choristers, dancers and stage managers -- voted 159 to 10 to reject the Met's contract proposal, which includes up to 41 1/2 percent in wage increases over a four-year period depending on inflation.

After the vote, Met board president Frank Taplin was asked if the opera season could still be salvaged. "I won't say. We're just going to continue to work" to seek an agreement, he said.

Traditionally at the Met, the settlement with the orchestra (which settled on Oct. 25) sets the standard for the other 15 unions at the theater -- a fact that bitterly offended the chorus, which for at least 20 years has sought salary parity with the musicians.