The fare on Metro's midday Downtowner bus will soon be raised to 25 cents a trip, members of a Metro committee agreed yesterday. But a board committee put off action on proposed fare increases and service cutbacks throughout the deficit-plagued transit system.

Since the Downtowner, which loops around downtown Washington, started seven years ago, its fare has been one of the best bargains in Washington -- just 10 cents for short trips through the F Street business district and 25 cents on longer hauls to Southwest and Washington Circle.

Although the low fares originally covered 75 percent of operating costs, last year they covered just 28 percent of the bill. Costs have soared primarily because of rising salaries. Ridership has dwindled because of the growth of the subway, and the Downtowner's annual deficit, paid for completely by the District of Columbia, rose to $247,000 last year.

Metro's revenue and operations committee voted unanimously yesterday to charge 25 cents for all Downtowner trips, a move that the full board of directors is expected to approve at its next meeting in early December.

Even though the increase is expected to bring in about $40,000 a year, D.C. Council member Jerry A. Moore, who serves as chairman of the revenue committee, said the District is likely to move ahead with a plan by Mayor Marion Barry to eliminate the Downtowner service next summer. A staff member said that the deficit is likely to rise to $350,000 in the current fiscal year and to $425,000 in fiscal 1982.

"That's just too much for us to handle," he said.

The proposed fare increase -- to a minimum of 60 cents in all parts of the area throughout the day -- won no praise, but provoked few protests.