The Metro board, seeking to end a four-year controversy, voted yesterday to build a six-mile extension of the subway system's long-stalled Green Line from Anacostia to a Prince George's County terminus near Branch Avenue.

The board's unanimous vote marked a reversal of a 1980 decision that called for a southern Prince George's route terminating near Rosecroft Raceway. Metro officials said they would ask a federal judge next week to lift a court order that has blocked the route's construction for nearly three years.

Under the board's plan, the southern branch of the Green Line is to reach from Anacostia to Congress Heights in Southeast Washington. A Southern Avenue station would be built at the D.C.-Prince George's border. There would be three stations in Prince George's -- at Naylor Road, Suitland and Branch Avenue.

The plan's advocates have argued that the route would improve transit service to St. Elizabeths Hospital, Greater Southeast Community Hospital and major employment centers in Prince George's, including a federal office complex at Suitland and Andrews Air Force Base.

It was uncertain yesterday whether the board's decision would face a court challenge by supporters of the Rosecroft plan, as some officials previously warned. In addition, no federal funds have been allocated for the route's construction, estimated to cost $483 million.

In a related move, Metro's board awarded a $25.6 million contract to a construction group to excavate subway tunnels for the Green Line under the Anacostia River after an investigation into allegations that a partner in the venture has ties to firms doing business in South Africa.

The award caused concern among board members partly because it coincided with nationwide protests against South Africa's policy of racial bias. Metro's investigation was triggered by complaints from the Maryland Minority Contractors Association and D.C. City Council member John L. Ray (D-At Large), an outspoken opponent of financial links to South Africa.

The Metro investigation concluded that Franki-Denys Inc., a minority partner in the construction group, has "very remote" ties with South African businesses. The company is partly controlled by a Belgian company that has subsidiaries in South Africa.

Metro officials said the board appeared to lack legal grounds to reject the construction group's bid, which was the lowest of 10 proposals received by the financially pressed transit authority. The bid was $13 million less than Metro officials had estimated the tunneling project would cost.

Ray said he would "hate to see a major contract" awarded to a company with ties to South Africa "even at this level," but added that the Metro board "didn't have any legal basis" to reject the bid. Ray praised the board for moving to draw up a policy to deal with future issues involving South Africa.

With the board's vote for the Branch Avenue route, the overall plan for the subway system was expanded to 102.7 miles. The Branch Avenue section is about 1.6 miles longer than the Rosecroft alignment. A total of 60.5 miles will be in operation Saturday when a Red Line extension to Shady Grove opens in Montgomery County.

The board's vote was applauded by Branch Avenue supporters, including Prince George's County Council member Sue V. Mills. "We hope the Rosecroft advocates will quietly go away at this point," Mills said. "They have threatened to take us back to court. If they go back to court, that's the end of any line."

"What's going to happen I don't know," said Joseph K. Burton, a community activist who supported the Rosecroft route. "That vote was sort of preordained when we had the last council election." The council voted 5 to 4 last year to support a shift from Rosecroft to Branch Avenue.

Metro planning officials also recommended a route for another controversial section of the Green Line through Northwest and Northeast Washington. The proposed section connecting the Columbia Heights and Fort Totten stations would be excavated mainly beneath New Hampshire Avenue. The board is expected to debate the proposal soon.