Ralph L. Stanley, the Reagan administration's mass transit chief, warned Metro officials yesterday that the White House is virtually certain to recommend cutting off all further federal spending for Metro subway construction.

The proposed cutoff, expected to be included in the president's budget for the next fiscal year, has threatened to disrupt the transit authority's plans to build several long-delayed subway extensions, including major parts of the Green Line, Metro's only unopened route.

Stanley's warning, in a hard-line speech to an annual conference of Metro officials, represented the second recent setback for the transit system. The Office of Management and Budget proposed last week halting funds for Metro as part of an effort to carry out landmark balanced-budget legislation.

"It is likely, if not certain, that the OMB recommendation is going to hold," Stanley told Metro and local government officials at the D.C. Convention Center. "That's a recommendation that is part of the president's budget that we'll support."

Later at an impromptu news conference, Stanley, who heads the Urban Mass Transportation Administration, sought to dispel reports from congressional sources that the Department of Transportation might seek to restore Metro's appropriation. "I would predict that it's going to be zero," he said.

Stanley's tough talk cast a pall over what had been planned as a festive session marked by two upbeat proposals by Metro General Manager Carmen E. Turner. Shortly before Stanley spoke, Turner announced plans to hold the line on Metro fares until mid-1987 and extend rail service to midnight on Sundays.

Turner's proposals, which had been disclosed last week, were included in a $458.8 million operating budget for Metro's 1987 fiscal year, which starts next July 1. At a budget briefing, Turner expressed dismay at the federal efforts to cut off construction funds.

"Our agenda is to build 103 miles of Metrorail in this region," she said.

The federal moves triggered protests from members of Metro's board of directors and pointed exchanges with Stanley. Gladys W. Mack, an aide to D.C. Mayor Marion Barry who heads the Metro board, termed the threatened cutoff "a serious breach of promise" that would deprive "economically disadvantaged" residents of rail service.

The Green Line, previously delayed by court challenges and controversies, is designed to serve low-income District neighborhoods, including Anacostia, along with parts of Prince George's County.

In his talk, Stanley attributed the proposed halt in Metro funds to the Gramm-Rudman-Hollings balanced-budget measure adopted by Congress last week.

Under the legislation, Stanley said, the Transportation Department can provide funds for Metro only if it reduces spending for other transit systems or for critical transportation programs, such as the nation's air traffic control system. "None of the choices are easy," he said.

At the same time, Stanley rejected pleas from Metro board members to release $216 million in federal construction funds appropriated by Congress last year. Metro officials have warned that any further holdup in these funds may delay a Red Line subway extension to Wheaton, scheduled to open in 1990.

Stanley said that Metro's plans for spending this allotment, along with an additional $227 million appropriated by Congress yesterday, must now be reconsidered in light of the Gramm-Rudman-Hollings measure. The review may take several months, he said.

According to Metro's estimates, the proposed cutoff would leave the transit system more than $700 million short of the funds it needs to expand the rail system to 89.5 miles by the early 1990s. The agency would face a nearly $3 billion shortage in its plan to complete 103 miles.

Stanley said it appears likely that Metro can finish about 76 miles, or "maybe a little bit more," under its current allotments. An earlier 76.4-mile plan called for building a section of the Green Line between Anacostia and a U Street station in Northwest Washington.