The Reagan administration has recommended spending $250 million a year for Metro subway construction for the next four years, apparently clearing the way for completion of an 89.5-mile rail system.

The plan, cited in an Office of Management and Budget document that was made available to The Washington Post, is the clearest signal so far that the administration may lift its controversial 76.4-mile limit on Metro construction.

Nevertheless, the OMB plan would bar further spending for Metro construction after 1989, apparently jeopardizing the transit authority's long-range goal of completing a subway network of more than 100 miles.

The OMB document, described as a 1986 "budget passback," or internal review, says that $250 million "will be provided per annum" until federal funds previously earmarked for Metro have been exhausted. It describes the proposed rail system as "89.5 miles."

The transit agency's board of directors endorsed a controversial proposal last fall to expand the current 60.5-mile subway system to 89.5 miles by tapping funds previously authorized by Congress.

The 89.5-mile plan, backed by Washington-area governments, includes a Green Line branch between Fort Totten in Northeast Washington and Greenbelt in Prince George's County and a Yellow Line spur to a Van Dorn Street station in Alexandria.

The OMB plan says, however, that "no further construction funds will be authorized" for Metro after the transit agency's remaining $1.2 billion congressional allotment is depleted.

The administration's opposition to further subway spending would throw into doubt Metro's current plan to expand the system to 102.7 miles by building another major Green Line branch between Anacostia and southern Prince George's, along with several other extensions.

Also in jeopardy would be a proposed Red Line terminus at Glenmont in Montgomery County, a planned Franconia-Springfield terminus in Fairfax County and a Green Line section connecting a U Street station in Northwest Washington with Fort Totten.

Metro officials have said they plan to seek a further authorization of federal funds from Congress to complete these extensions.

The authority previously had endorsed a 101-mile system, but the plan was expanded to 102.7 miles last month when the Metro board revised its proposed alignment for the Green Line in southern Prince George's to include a terminus near Branch Avenue.

The board's endorsement of the Branch Avenue plan, which replaced a shorter route with a terminus near Rosecroft Raceway, also increased the cost of completing the proposed rail system by more than $130 million. Metro officials previously estimated the 101-mile system at more than $9.2 billion. A revised estimate has not been drawn up.

An OMB spokesman declined to comment on the administration's recommendations. Other federal and local officials said that the OMB plan for spending $250 million a year for Metro appeared highly significant because it matched proposals by the transit authority and by the federal Urban Mass Transportation Administration.

Officials said, however, that the federal transit agency has not completed its review of the 89.5-mile plan. One key issue is expected to center on federal demands for a contract guaranteeing completion of 89.5 miles within available funds -- a proposal considered controversial because of Metro's 102.7-mile goal.

Nonetheless, Ralph L. Stanley, the federal agency's administrator, said in an interview last month that he believed the 89.5-mile plan "looks fine in principle." But he added, "I've got to go to OMB with it . . . . It would be very premature to say that we've approved it."

Stanley could not be reached yesterday for comment. Other officials said it is unclear whether OMB's response had removed Stanley's uncertainties about the plan.

The OMB plan for Metro spending also appeared in line with recent congressional actions. Last year, Congress approved an amendment to a spending measure designed to force the Reagan administration to lift its 76.4-mile limit on Metro construction.

Although the OMB plan would provide substantial funds for Metro construction, the agency's overall recommendations would result in major cutbacks in other forms of financial aid for Metro and other transit systems throughout the United States. These proposals are expected to face stiff opposition in Congress.

The plan would eliminate federal operating subsidies for transit systems, including $18.5 million a year for Metro. It would cut federal funds for building, modernizing and buying equipment for bus and rail systems from nearly $3 billion a year to $1.1 billion. Transit systems would have to provide a larger local share to "match" federal grants -- 50 percent instead of 25 percent.