A federal judge, setting aside a recent congressional measure, ruled yesterday that Metro may not be sued by workers who have been injured at subway construction projects.

The decision, by U.S. District Court Judge John H. Pratt, marked a major shift in a multimillion-dollar dispute that has caused concern among Metro and other local officials. A Metro spokesman hailed the ruling, saying, "It sounds very good."

William F. Mulroney, a lawyer representing the injured construction workers, said he plans to appeal Pratt's ruling to the U.S. Court of Appeals. The decision blocked claims by 96 workers, many of whom suffered lung ailments at tunnel excavation sites, according to lawyers for the workers.

In rejecting the workers' claims, Pratt cited a recent U.S. Supreme Court ruling that had been described as a victory by Metro officials. The high court ruled in June that Metro is immune from lawsuits by injured construction workers.

In September, Congress adopted an amendment aimed at nullifying the high court decision. Pratt held, however, that the congressional measure does not apply to lawsuits by Metro construction workers.

The amount of money involved in the lawsuits has been a matter of dispute. Metro officials initially contended that the claims totaled $3.5 billion and warned that these costs might pose "serious financial problems" for the transit system, including possible delays in construction of subway extensions.

In September, however, lawyers for the workers offered to settle the lawsuits for a total of $13.5 million. It was unclear yesterday what impact Pratt's ruling would have on negotiations over a possible settlement.

In his decision, Pratt said that the congressional amendment did not apply to Metro workers because of a D.C. Council measure, which took effect two years ago.

Until 1982, workers' compensation proceedings in the District, including claims by Metro construction workers, were regulated under the federal Longshoremen's and Harbor Workers' Compensation Act. The District measure replaced the longshoremen's act with new D.C. procedures for workers' compensation.

Since the D.C. City Council had repealed a law linking local compensation proceedings to the federal longshoremen's act, Pratt said, claims by Metro workers would no longer be affected by congressional revisions of the longshoremen's act, including the recent amendment aimed at countering the Supreme Court's June decision.

The congressional amendments "do not reach these District of Columbia cases," Pratt said.