The Metro board postponed yesterday final approval of fare increases for the bus and subway system because the Fairfax County Board of Supervisors found them unacceptably high.

The deferral means that the fare increases, if they come, will probably go into effect after July 1, the date they were originally scheduled.

That is because it will take time to reprogram the computers at every fare gate in the Metro subway system with a new fare table.

Also deferred with the fare package were final approval for extended nighttime and weekend subway hours and the implementation of two-week transit passes that could be used for both subways and bus rides.

There has been solid regionwide support for both the transit passes and extended subway hours and those items do not appear to be imperiled.

However, the Metro board's formal action yesterday was to return the entire fare schedule to its Revenue and Operations Committee, thus theoretically reopening all questions. The committee had reported out a compromise proposal last week after many difficult meetings.

Fairfax County Supervisor Joseph Alexander, a member of the Metro board, tried to avoid returning the entire schedule to committee yesterday and asked the full Metro board to approve the rail fares and the transit passes, but to give Northern Virginia a week to work on bus fares.

That was unacceptable to both the District of Columbia and Maryland representatives to the Metro board, who insisted that part of the package could not be reopened without reexamining the whole.

The heart of the problem is that the Metro does not pay for itself from either the bus or the subway side. The difference between the costs of operating the transit system and the revenues collected in fares are divided among area jurisdictions by a complex set of formula.

Thus, if any action lowers the amount of fares collected, it increases the tax support necessary to make up the difference. Participants in Metro fare negotiations often try to see how much of the subsidy can be shifted from one jurisdiction to another. All jurisdictions want to be certain they are treated fairly.

The situation gets more complicated in Maryland and Virginia, where the deficit is suballocated to various cities and countries.

The Northern Virginia Transportation Commission, an agency made up of the Northern Virginia partners in Metro, met last night to work out a new Virginia fare proposal for next week's Metro board committee meeting. The Metro board has final fare authority.

The Fairfax County Board on Monday asked for a bus fare restudy after learning that Metrobus had lost about 20,000 riders a day immediately after a large fare increase in March 1977.

"We are concerned that revenue gains will be completedly offset by the loss in ridership." Alexander told the Metro board. Fare increases for Virginia riders to downtown Washington, using bus and rail, would average about 15 cents a one-way trip under the proposed fare increase.