The chairman of the New York State Mediation Board said today he will seek to have New York state's commissioner of labor invoke a rarely used provision of state law to impanel a special board of inquiry--a blue-ribbon, nonpartisan group to investigate the bitter 6-week-old New York City Opera strike.

"In my judgment, mediation is not going to be successful in this dispute," New York state's chief mediator said today. He described the panel as a last-ditch attempt at a solution.

Union leaders immediately said the inquiries seemed unlikely to provide orchestra members with the fair settlement they need, and therefore, according to musician's union president John Glasel, "We're not going to cooperate with the board of inquiry."

Meanwhile, NYCO director Beverly Sills told the musicians that the opera company's performances through Sept. 15 are canceled. Sills also said that if the strike continues through Aug. 24, additional future performances will be scratched. The opera company's new summer-autumn season was to begin July 7 and run 19 weeks--through mid-November.

The Rev. Msgr. James A. Healy, chairman of the Mediation Board, said, "We feel there is such an enormous public interest" in settling the opera vs. musicians dispute "that we have to take this extreme measure."

The last time such a board of inquiry was convened, Healy said, was about 10 years ago.

Such a panel cannot force the feuding sides to accept its findings and terms of its settlement recommendations, but the weight of such a public body's observations carries major impact in this state, Healy said. The panel is to be composed of well-known citizens who have yet to be named.

Binding arbitration has already been suggested by NYCO and has been rejected by the musicians union, which said its position on the issue is based on a previous, unsatisfactory experience with arbitration.