An arbitrator yesterday approved Metro's plans to lay off bus drivers for the first time in the eight-year operating history of the public transit authority.

The ruling came on a challenge to Metro from Local 689 of the Amalgamated Transit Union, which represents Metro's 2,700 bus drivers. Metro officials said late yesterday that 35 full-time drivers and 12 part-time drivers will lose their jobs as a result of service cuts that take effect Sunday. Metro officials said the effective date of the layoffs has not been decided, but it probably will be within two weeks.

Arbitrator Richard I. Bloch disagreed with the union's contention that Metro was obliged by its contract with Local 689 to retain all full-time drivers until all part-timers had been fired. Bloch ruled that Metro can retain part-timers, but that their percentage cannot exceed the percentage of part-timers that Metro had when the layoffs begin.

Metro's present part-time force is 8.8 percent of the full-time force; thus 8.8 percent will be the ceiling. The contract permits Metro to have a part-time force equal to 10 percent of the full-time force. Bloch's ruling is final and binding.

The layoffs will come as Metro cuts back some bus schedules and takes advantage of a new work rule it won during a recent contract negotiation that permits a different scheduling system. Since January, Metro General Manager Richard S. Page said yesterday, a combination of service cuts and tighter management procedures has resulted in a reduction of 148 bus drivers. Attrition has taken care of all those cuts to date, Page said, but the attrition rate has slowed in recent months.

The newest drivers will be the ones laid off under seniority rules, and full-time drivers who are laid off may "bump down" to part-time positions if they wish. Page said Metro "will make every effort" to consider for other Metro jobs those drivers who are laid off.