The U.S. Department of Transportation's civil rights office has told Washington Metro that the controversial subway line to Rosecroft Raceway could have an unfair, negative impact on two black communities near which the line would run.

That "preliminary judgment," presented in a letter to Metro acquired by The Washington Post yesterday, provides yet another roadblock to the completion of the Rosecroft line, already stalled by both a court ruling and the uncertain availability of federal dollars. Technically, the federal government could withhold funds for construction of the Green Line if a civil rights problem were proved.

The letter, dated March 6, came as the result of a complaint filed last May by Ronald Hill, a resident of the Hillcrest Heights subdivision in Prince George's County, not far from the District of Columbia line. According to Metro documents, the tracks at that point would be "approximately 100 feet from the homes in the Hillcrest Heights subdivision at window level and 200 feet from homes in the Woods subdivision." The elevated structure carrying the tracks would be above a creek bed that separates the two subdivisions.

The Department of Transportation letter said the route could create "an imbalanced and disproportionate environmental influence on the black community as compared with the white communities in the surrounding areas." Furthermore, the letter said, the reasons for choosing the particular routing "appear inadequate as justification for such extreme adverse impacts. . . ."

Metro General Manager Richard S. Page had no comment. However, Metro's formal answer to the letter, also obtained yesterday, asked that the civil rights office give Metro "a statement of the findings of your investigation which led to the conclusions. . . ."

Linda Gosden, press secretary for the department, said the purpose of the letter "was to advise Metro of the problems that could arise if the project progressed" and that the nine months between Hill's complaint and the letter was about normal for a project scheduled far in the future.

No investigation was conducted, she said, and "no action is pending." She said "absolutely not," when asked if the civil rights letter is a back-door attempt by the department to eliminate the Rosecroft Raceway segment of the Green Line.

Both Metro and department officials said that they will meet soon to discuss the issue.

The inner-city section of the Green Line, from Anacostia to 14th and U streets NW, would not appear to be affected by the complaint. The Metro board, faced with limited funds in the Reagan budget and a number of lines ready for construction, is trying to reach agreement on how to schedule construction of that section, which the District government is demanding.

The Reagan administration's budget calls for a slower rate of construction than Metro has scheduled, and the section from Anacostia south to Rosecroft is already one of the last in the planned 101-mile Metro system.

The schedule says it would open in 1989, but it is generally agreed that 1991 is a more realistic date.From the planned Anacostia station, at Howard Road and Interstate 295, the Rosecroft line would run generally parallel to Wheeler Road with stops at St. Elizabeths Hospital, St. Barnabas Road and Rosecroft Raceway.

The route is controversial because it is a change from a long-planned route that would have gone from Anacostia east into Prince George's County to the Suitland Federal Center and then to a business community near Branch Avenue and Auth Road. Citizens and businessmen in the Branch Avenue community have sued Metro because of that change, which was directed by the Prince George's County Council.

Metro was told by a federal court in Baltimore that the change was illegal because a proper public hearing had not been held. Metro has appealed the ruling, but is also planning the public hearing, probably sometime in June. In the meantime, the county council has reaffirmed its affection for the Rosecroft route.