Metro officials said yesterday they will have to cancel plans to begin $120 million worth of construction on the Anacostia portion of the long-delayed Green Line in the next year, heightening concern that much of that subway line might never be built.

The announcement came one day after a federal judge in Baltimore barred Metro from spending any money on the Green Line's southern half if the route deviates from the system's 1968 master plan, as much of Metro's planned routing currently does.

To salvage construction timetables, officials said, Metro will consider shifting Anacostia funds to the northern portion of the Green Line. The line is supposed to run from Greenbelt in Prince George's County through the District's urban core, across the Anacostia River and into southern Prince George's.

Metro officials believe the decision, by Judge Norman P. Ramsey of the U.S. District Court in Baltimore, freezes everything they had planned from the Navy Yard station south to Rosecroft in Prince George's. To build the lines where Metro wants them, new public hearings will have to be convened, pushing schedules back at least a year. Further legal challenges could lengthen that delay.

Metro had planned to advertise later this month for a $60 million contract to tunnel under the Anacostia River, the first major spending in years on the so-called inner city Green Line. Late this year, it planned to seek a contractor to build the Anacostia station itself, another $60-million job. Some $100 million was planned for fiscal 1983 to build the Navy Yard station.

Metro General Manager Richard Page, calling the court's decision "almost nonappealable," said plans to advertise the tunnel job would be canceled. He said Metro would seek permission to continue small-scale contracts that are already under way, such as a design contract for the Navy Yard station.

Page said Metro would consider transferring the funds to begin work on the Green Line's Mount Vernon Square-UDC station at Seventh and M streets NW ahead of schedule. Metro had planned initiate construction there in fiscal 1984.

However, Anacostia has a higher priority than Mount Vernon Square and, with federal funds shrinking, D.C. officials say they may want to reserve the money for Anacostia instead of shifting it.

To date, almost none of the Green Line has been built. D.C. Mayor Marion Barry and Prince George's County officials have placed heavy emphasis on getting construction started because they fear that every new delay increases the chance that parts or all of the line will be canceled.

The 1968 plans shows the route crossing the Anacostia River by the 11th Street Bridge, moving southeast under Good Hope Road, surfacing just inside the D.C. line, then running along Suitland Parkway to a terminus at Branch Avenue.

Early in 1980, following public hearings, the Metro board officially redrew the route. The crossing site was moved downstream slightly, and the route was set to run more directly south and terminate at the Rosecroft Raceway. This route would ease disruption and relocation in D.C. and better meet the transit needs of Prince George's County, it was argued.

The shift led a group of Prince George's citizens to file suit in federal court in Baltimore in May 1980 charging Metro had improperly changed the routing to Rosecroft. In February 1981, the court ruled that Metro did not correctly advertise the Prince George's County hearings about the change. The court reserved action until appeals were completed.

Last October, Metro reconvened hearings in Prince George's County as a precaution--no new hearings were held in the District, however. "We interpreted the suit and the court actions to be confined to Prince George's County," said Metro attorney Jerome Jani.

Last November, the U.S. 4th Circuit Court of Appeals in Richmond upheld the Baltimore court's decision and referred the case back to it for action, which Judge Ramsey took on Tuesday. However, he claimed authority over District portions of the line as well, and ordered that all work stop.

(Lawyers say that the same technical errors that Ramsey faulted in the Prince George's hearing advertisements occur in the District advertisements. A separate suit was filed in the U.S. District Court in D.C.)

Metro believes that the new hearings in Prince George's last fall have met Ramsey's requirement for the county. However, new hearings will have to be convened in D.C., they say. They are tentatively scheduled for June this year, after which a report might go to the board in the fall.

That would mean a minimum of a year lost. But assuming that Metro decides again for the Rosecroft alignment, proponents of the Branch Avenue route may well file new lawsuits. Lawyers for businessmen who brought the Prince George's suit against Metro say they will tie it up indefinitely in new litigation if Metro does not restore the Branch Avenue alignment.

General Manager Page said that the court's decision "is going to be a very costly setback to the construction of any subway line to any destination in Prince George's County." Current cost estimates for the southern portion of the Green Line are about $750 million, he said, with the price rising 10 to 12 percent per year of delay.