A Senate subcommittee voted yesterday to reduce federal spending for Metro subway construction by 25 percent next year, jeopardizing the transit authority's long-debated plan to expand the rail system to 89.5 miles by the early 1990s.

The action, by the transportation subcommittee of the Senate Appropriations Committee, set Metro's allotment for fiscal 1986 at $187.5 million, markedly less than the $250 million sought by Metro officials. The Reagan administration also had initially recommended the $250 million level.

The cut triggered immediate objections from Virginia and Maryland members of Congress, concerned about endangering the 89.5-mile plan. The subway construction plan includes a Green Line extension to Greenbelt in Prince George's County and a Yellow Line spur to Alexandria's West End.

The House recently approved a $237.5 million appropriation for Metro, a 5 percent cutback that congressional aides said the transit agency "could live with." If the Senate endorses the Appropriations subcommittee's measure, the issue will have to be settled by a Senate-House conference committee.

"What I'm worried about is the [89.5-mile] agreement," said Sen. John W. Warner (R-Va.). If the Senate refuses to accede to the House measure, he said, "it would be a devastating shortfall" for Metro.

While sharply reducing Metro's allotment, the subcommittee voted to freeze operating subsidies for the nation's transit systems at current levels and to impose more modest cuts, generally less than 15 percent, on most other transit spending.

Under the subcommittee measure, Metro would benefit from the freeze in operating aid and other shifts in federal spending. Metro now receives $18.5 million a year in federal operating subsidies, along with millions of dollars in aid to buy buses and equipment, and build garages.