A multibillion-dollar fiscal crisis has threatened to delay and possibly derail Metro's plan for completing its 103-mile subway system, raising renewed concern about prospects for the long-stalled Green Line and other proposed rail extensions in the District, Maryland and Virginia.

In the short run, transit officials say, cutbacks in federal funds could lead to delays in opening two major sections of the Green Line in Southeast and Northwest Washington, as well as in northern Prince George's County.

A slowdown in federal grants also may set back a Red Line extension to Wheaton, transit officials say.

In the long term, four other extensions are imperiled by an estimated $2 billion shortage of funds. These include Green Line segments in the District and southern Prince George's, a Red Line spur to Glenmont in Montgomery County and a Yellow Line branch to a Franconia-Springfield station in Fairfax County.

Metro's financial troubles, the latest in a series of crises that have gripped the transit authority since the late 1960s, are an outgrowth of long-term cost overruns in building the subway system and stepped-up efforts by Congress and the Reagan administration to curtail spending.

The cost of completing the 103-mile system, now estimated at more than $9 billion, has nearly quadrupled since 1968, when it was put at $2.5 billion.

The increases reflect high inflation rates in the 1970s and early 1980s, delays in construction, and other factors.

The administration, worried by mushrooming transit costs across the nation, is seeking to limit spending for Metro and other transit systems.

Most of Metro's construction funds have come from the federal government. Congress also is moving to reduce outlays for transit as part of an overall effort to curb the nation's deficits.

Ralph L. Stanley, the administration's mass transit chief, has urged Washington area officials to take action to deal with the "funding realities" confronting Metro in an era of fiscal restraint.

"You can postpone it, but you can't really avoid it," he warned.

John S. Egbert, Metro's deputy general manager and construction chief, has described Congress' latest move to slash spending for Metro as "a back-breaker."

Egbert said major cutbacks could force local officials to revise their overall construction plans.

"If they continue to do this, we'll never get the system finished," added Fairfax County Supervisor Joseph Alexander, a veteran member of Metro's board of directors.

The federal government's outlook on mass transit has changed markedly since 1979, when Congress last bailed out Metro by putting up an additional $1.7 billion to complete the system.

The nation then faced widespread gasoline shortages, and public transit was touted as a key solution to the energy crisis.

Now, with gasoline prices relatively steady and mass transit under attack for excessive costs, no similar bailout is in sight. The administration opposes any new allotment for Metro, and congressional aides say that Metro faces an uphill battle for federal funds.

Despite its recurrent fiscal setbacks, the nine-year-old subway system already encompasses60.5 miles, and a nine-mile Orange Line extension from Arlington County's Ballston station to a Vienna terminus in Fairfax County is to open next summer.

Metro officials say this extension is not endangered by the current fiscal problems because its construction virtually is complete.

In many other respects, the rail system has shown steady gains and drawn widespread praise. Subway ridership has reached record levels, the reliability of rail service has improved, and safety standards have been tightened.

But construction costs remain the chief problem.

Much of the current concern centers on the Green Line, the only unopened route in the Metro system.

The Green Line is designed to serve Anacostia and other low-income areas, where many residents depend heavily on public transportation. The line also is viewed as a major link to Prince George's.

Despite repeated pleas from federal and local officials, construction of the Green Line was delayed by lengthy court battles and neighborhood opposition in the District and in Prince George's.

These disputes now appear to have been settled, and construction recently started in Anacostia and other areas.

Three key developments have helped fuel Metro's current crisis:

*The Senate has voted to reduce spending for Metro construction by 26 percent this year to $184.5 million, the lowest level since the transit agency was embroiled in a similar fiscal controversy eight years ago.

*The administration is holding up federal grants to put pressure on Metro to conclude negotiations on a contract aimed at regulating construction spending.

*Although local officials have known for five years that Metro would need more money, no action has been taken to bridge the expected $2 billion gap. Local officials say they want another congressional allotment, but some key federal officials say local governments should pay a major share of the costs.

According to Metro officials, a sizable cut by Congress in this year's appropriation probably would delay construction on two sections of the Green Line, where contracts for more than $100 million in subway projects are expected to be awarded in coming months.

One of these sections extends from Anacostia to a planned U Street station near 13th Street NW. It currently is scheduled to open in 1990.

The other section, planned to open in 1992, would connect a proposed Greenbelt terminus in Prince George's with the Fort Totten stop in Northeast Washington.

The House previously had recommended cutting Metro's appropriation by 5 percent to $237.5 million.

Senate and House conferees are expected to weigh the issue soon, and Metro's advocates say they will urge the conferees to adopt the House recommendation in an attempt to avoid disruption of construction plans.

Because of the negotiations with Metro, federal officials have refused to release millions of dollars appropriated last year by Congress. If the grants are withheld beyond this month, Metro officials contend, work may be delayed on the Red Line extension to Wheaton, now projected to open early in 1990.

The contract negotiations are closely linked to congressional actions on outlays for Metro.

The contract is expected to spell out procedures under which Metro can expand the system to89.5 miles by tapping about $1 billion in federal funds remaining from Congress' $1.7 billion authorization.

Because Metro is counting on appropriations of about $250 million a year to complete the89.5 miles, any major cuts by Congress would raise doubts as to whether Metro could carry out the terms of the proposed contract.

In the protracted negotiations, federal officials are seeking relatively stiff terms to put a lid on federal spending for Metro.

If federal funds run out before the 89.5 miles are completed, they argue, Metro will have to seek more money from local governments to finish the work, or settle for a smaller system.

In addition, federal officials say, they want to prevent Metro from starting construction on any section for which insufficient funds are available.

If any project is left unfinished, they say, Metro will have to reimburse the federal government for wasted money.

Metro officials, concerned about possible cutbacks by Congress and future increases in inflation, are seeking escape clauses in the contract.

If funds are exhausted because of spending cuts or inflation, they say, Metro should be allowed to seek further federal aid to complete the 89.5 miles.

In addition to the Green and Red Line extensions, the 89.5-mile plan includes a Yellow Line spur from Alexandria's King Street station to a Van Dorn Street stop in the city's West End.

This branch also could be delayed if congressional outlays are cut in the next few years, Metro officials say.

Even if Metro completes the89.5 miles under terms of the proposed contract, the transit agency is expected to need another $2 billion to build the remaining13.5 miles. Without these additional sections, Metro officials contend, passengers and local governments would face major problems.

One of the unfunded sections would link the Green Line's U Street station with Fort Totten, providing service to Columbia Heights and Georgia Avenue NW.

Without this segment, the Green Line would consist of two unconnected routes. Prince George's riders would have to transfer to the Red Line at Fort Totten.

Another currently unfunded section would extend from Anacostia to a Branch Avenue terminus in Prince George's. The extension is needed, officials say, to serve Southeast Washington communities such as Congress Heights and the southern part of the county.

Also unfunded are the Red Line's Glenmont extension and the Yellow Line's Franconia-Springfield branch.

These stations are needed, officials say, to provide service and additional parking spaces for Montgomery and Fairfax commuters and to ease traffic congestion. In addition, a rail yard is planned at Glenmont.