The chairman of the Washington Metropolitan Area Transit Commission yesterday rejected Metro's attempt to block the start of Fairfax City's new chartered bus service for commuters, clearing the way for it to start operation on schedule next Tuesday.

After a 45-minute hearing into the question, William R. Stratton, the commission chairman, said Metro's case for a delay was so weak that he was able to reach his decision immediately.

Although Stratton cannot act alone, the concurrence of a second member is expected almost routinely today, to be followed by the issuance of a formal order. The commission is a regulatory body that oversees the operations of privately owned bus companies in the region. Metro, as a public agency, is exempt from its regulation.

The Fairfax City Council voted last month to stop paying subsidies Dec. 31 for Metrobus service that is now operated through the community. As a replacement, the city has hired the Gray Line, Inc., to provide and operate seven buses each weekday between Fairfax City, the Pentagon and downtown Washington.

To start the service, Gray Line had to obtain the transit commission's approval for the price it will charge Fairfax City for hiring the buses. The Metro board of directors voted last Thursday to ask the commission to delay the approval - and the start of the chartered bus service - at least until Feb. 1.

At the time, some Metro board members said the delay would minimize the publicly owned transit system's operating deficits while the board decides how much service, if any, it may curtail in Fairfax City.

Fairfax City, which is politically separate from surrounding Fairfax County, is served by eight Metrobus routes. Most of those routes go to destinations in the county, chiefly serving county residents. Metro is seeking dropping some of the existing service, and also the possibility of not stopping buses in the city to pick up or discharge passengers.

Jordan S. Himelfarb, a Metro lawyer, did not explain Metro's motive in seeking to block the city-chartered service yesterday, other than explaining that the Metro board needs the time to hold public hearings and decide on its course of action.

L. C. Major, lawyer for Gray Line, and Fairfax City attorney John Rust, said the new service would be harmed by a delay. Major said Gray Line bought nine new buses in anticipation of running the new service, and these would remain idle until a decision is reached.

Stratton, in announcing his decision, criticized Metro for creating "a policy vacuum," and said it should reach a clearly stated policy before seeking commission rulings in such matters.