A Metro board committee recommended yesterday that Metro operate trains until midnight instead of 11 p.m. when the system expands its hours to week nights and Saturdays beginning Sept. 25.

The committee's unanimous recommendation is virtually certain of approval by the full Metro board next week. The added hours were proposed by the panel after many citizens testified at recent public hearings that 11 p.m. was too early to close for people using the subway for evening outings.

The committee also asked for a study of possible limited Sunday service for seven or eight hours late in September.

Furthermore, the committee proposed a reduction in bus and subway fare increases scheduled to take effect July 1. The final version of that fare package will not be known for at least another week since several points remain in negotiation.

It is known, however, that there will be no change in District of Columbia bus fares. There will be increases for suburban riders.

It is also known that a subway trip during non-rush-hour periods will cost all riders 50 cents, no matter where they get on or where they get off in Washington, Maryland and Virginia.

The 50-cent flat fare will apply between rush hours on Weekdays, after 6:30 p.m. week nights and all day Saturday and Sunday. Thus, after Sept. 25, it will be possible for 50 cents to travel to the Mall from Silver Spring or to ride from Crystal City to RFK Stadium on a Saturday afternoon. Such trips today during rush hour would cost 80 cents and 65 cents, respectively.

A sliding fare schedule based on distance traveled and other factors will continue to be in effect during rush-hour periods between 6 a.m. and 9:30 a.m. and between 3 p.m. and 6:30 p.m. weekdays. Exact station-to-station fares still must be computed in many cases.

Metro's trains presently run on weekdays only between 6 a.m. and 8 p.m. Under the schedule proposed yesterday, weekday trains will run from 6 a.m. to midnight and Saturday trains will run 8 a.m. to midnight. The hours for possible Sunday service are undecided.

"There will be extensive overtime necessary to run the six-day schedule," Metro board member Francis B. Francois said. "We asked for a study of Sunday service to see what would happen if we added a seventh day. We may be able to reduce some overtime by adding a full Sunday shift.

Sunday service would also provide subway service to RFK Stadium for Redskin football games. A group of downtown Washington churches has also asked for Sunday subway service.

After Sept. 25, the day Metro plans to expand subway hours, six Redskin home games remain. Four are on Sundays, one is a Monday night and the other is on a Saturday.

Earlier expansion of service is not possible because the night-time and weekend hours are needed for testing operations and break-in runs on new Metro cars, according to Metro officials.

The extra hour for extended week night and Saturday service will cost about $150,000 in additional subsidies for fiscal 1979 from area governments, Metro estimates. More detailed financial analyses of various proposals, including Sunday service, will be completed next week.

The new fare package will also include, for the first time, two-week transit passes. There are three types:

A $10 pass, which will include $5 worth of subway rides and an unlimited number of bus rides within one bus zone, including the entire District of Columbia.

A $16 Maryland-District pass. It would tentatively contain $6.50 worth of subway rides and would be good for the full fare of any transit trip between Maryland and the District.

A $16 Virginia two-zone pass. It would contain $5 worth or subway rides and would be good for any two-zone bus trip within Virginia.