General Manager Carmen E. Turner has told the Metro board that she plans to hold the line on bus and subway fares until mid-1987.

Under her plan, fares would remain frozen for a third year, the longest stretch the transit authority has ever gone without a fare increase. Fares were last raised in June 1984. Fares would be held at their current level at least until June 30, 1987, the end of Metro's next fiscal year.

Turner, who has sought to keep down fares to bolster ridership, outlined her plan at a closed board meeting last week and is scheduled to announce it next week along with an operating budget of more than $400 million for fiscal 1987.

Turner also is expected to propose extending rail service until midnight on Sundays.

Board members expressed broad support for holding the line on fares. "It's essential that Metro do everything it can to keep fares down," said Cleatus E. Barnett, a Montgomery County representative who is expected to become chairman of the six-member Metro board next month.

"Right now we are in agreement that fares won't go up," said D.C. City Council member Hilda H.M. Mason. Similar views were expressed by Fairfax County Supervisor Joseph Alexander, Prince George's County Council member Richard J. Castaldi and Arlington County Board member Mary Margaret Whipple.

Gladys W. Mack, an aide to D.C. Mayor Marion Barry who now heads the Metro board, declined to comment on Turner's plan, but said, "We do have a goal of keeping fares low and cutting costs."

From 1977 to 1984, Metro fares went up almost annually. Minimum subway fares doubled from 40 cents in 1977 to 80 cents, while basic rush-hour bus fares rose from 50 cents to 80 cents. The authority also increased surcharges for long subway rides and bus trips that cross transit zones.

Last year, Turner adopted a hold-the-line stand on fares, saying she wanted to keep rates at an "affordable" level to encourage ridership. Her new plan is designed to extend this strategy for at least another year.

Board members attributed the authority's ability to maintain the current fares to relatively low inflation, increasing ridership and cost-cutting steps. The hold-the-line strategy appears significant because the rail system is scheduled to open an Orange Line extension to Vienna in June, a costly move.

This year, ridership climbed to the highest level in 30 years, surpassing a mark set during the nationwide gasoline crisis in 1979 and 1980. Ridership reached a level that was last recorded at the end of the Korean War, when the Washington area was served by privately owned bus and streetcar companies.

"By holding fares steady this past year, ridership improved, and that's good for the system," said Whipple. "Ridership has been at an all-time high."

In the past, proposals to increase Metro fares have repeatedly sparked controversies. City officials have expressed concern about the cost to low-income riders who depend heavily on public transportation, while suburban officials have objected to high rail fares for long rush-hour trips.

The plan to avoid a fare increase faces several uncertainties, officials said. These include efforts by the Reagan administration to reduce federal operating subsidies for mass transit and negotiations over a new contract with the Amalgamated Transit Union, which represents most Metro employes.

Congress previously has balked at any sizable cut in the subsidies, which now provide $18.5 million a year for Metro. Metro officials are seeking to hold down wages and cut costs by modifying other contract provisions in the labor negotiations, set to begin soon. The union is pushing for higher wages and benefits.

Turner's proposal to extend rail service from 6 p.m. to midnight on Sundays was initially included in a series of preliminary budget recommendations last June.