The Supreme Court, ducking one of the most controversial cases before it, declined yesterday to decide the constitutionality of a New Jersey law requiring a daily "moment of silence" in public schools.

The court, in a decision focused solely on procedural legal questions, ruled unanimously that state lawmakers favoring the law had no legal authority to present their case to the high court.

The action left intact a lower court ruling striking down the 1982 law as a violation of the First Amendment's required separation of church and state. But the decision gave no guidance on the constitutionality of similar "moment-of-silence" laws in about two dozen states.

In other action yesterday, the high court sharply curtailed the ability of environmental groups to sue polluters under provisions of the 1972 Clean Water Act, ruling that groups may sue only for continuing, not past, violations of the law.

The unanimous decision, a major victory for business groups, came in a case involving discharges into the Pagan River in Smithfield, Va., by a meat-packing plant owned by Gwaltney of Smithfield. A federal appeals court had upheld a $1.3 million civil penalty against Gwaltney, largest penalty ever imposed in a citizen enforcement case under the act.

But the high court, in an opinion by Justice Thurgood Marshall, said Congress, in allowing private "citizen suits" to supplement government enforcement actions, only permits suits "to enjoin or otherwise abate an ongoing violation." Congress wanted to encourage citizen suits that were "primarily forward-looking," he said.

Environmental groups argued that Congress intended citizen suits for past violations as a potent weapon against polluters, especially at a time of lax state and federal enforcement of federal antipollution laws.

But Marshall said the citizen suits are "meant to supplement rather than to supplant" government enforcement, and that to allow such suits could interfere with the law's purpose of encouraging compliance through settlements.

Gwaltney, sued by two environmental groups, argued that it had ceased violating its permitted pollution levels before the suit was filed, although after the groups notified the company that they intended to sue.

E. Barrett Prettyman Jr., an attorney for Gwaltney, said yesterday he was "extremely pleased by the court's decision," which would prohibit suits for pollution violations that were "over and done with, cured," before litigation commenced.

Marshall sent the case, Gwaltney of Smithfield v. Chesapeake Bay Foundation Inc. and Natural Resources Defense Council, back to the appeals court for further review.

The New Jersey school case, Karcher v. May, returned the court to a recurring battle over religious exercises in public schools. The court in 1985 struck down an Alabama law requiring a moment of silence for "meditation or prayer," saying that the law was passed as a means of circumventing the 1962 decision banning organized prayer in schools.

But a majority in that case said they would uphold a neutral law, one that did not mention prayer or was not intended to promote religion. The New Jersey law required a "one-minute period of silence" for "quiet and private contemplation or introspection."

The law was challenged shortly after it was passed over the governor's veto and the state attorney general, two boards of education and the state education department refused to defend it.

As a result, Alan J. Karcher, speaker of the general assembly, and senate President Carmen A. Orechio intervened in the case to represent the state legislature. Both men were replaced in their leadership positions shortly after the appeals court's ruling and the new state legislature declined to appeal. But Karcher and Orechio did so in March 1986.

Justice Sandra Day O'Connor, writing for the court, said that Karcher and Orechio had no legal standing to pursue the case once they lost their official positions in the legislature. "The controversy ended when the losing party -- the New Jersey Legislature -- declined to pursue its appeal," she said.