A D.C. Superior Court judge ruled yesterday that the three-year-old City Council regulation prohibiting possession of five commonly abused drugs - L.S.D, P.C.P., Preludin, quaaludes and Ritalin - is uneforceable because it did not specifically state why the drugs are dangerous.

The U.S. Attorney's Office immediately petitioned the D.C. Court of Appeals for an early hearing on the decision by Judge Tim Murphy, which now affects only cases he hears.

If the judge's ruling is uphel on appeal, however, it potentially could invalidate the majority of more than 300 narcotic prosecutions now pending in Superior Court and call into question hundreds of drug convictions obtained since the regulation became effective in 1975.

Unathorized possession of the five drugs would remain illegal in the District of Columbia under the federal Controlled Substances Act, even if the appeals court supports Murphy's finding, according to the U.S. Attorney's Office.However, considerable legal confusion could result.

When the D.C. City Council approved the regulation in 1975, it intended to incorporate the five drugs into the Dangerous Drug Act of the D.C. Code, so that violations could be prosecuted in D.C. Superior Court, rather than in U.S. District Court, prosecutors said.

If the appeals court found that the regulation was unenforceable, defendants charged with possession would have to be prosecuted in the federal court, prosecutors said. The change could result in massive administrative switching jurisdiction over the narcotics cases, lawyers said yesterday.

It is possible that the Council could act immediately to correct the defect Murphy found in the regulation, and thus avoid the necessity of transferring the cases to the federal court, prosecutors said.

The appeals court is scheduled to hear oral arguments on the question this afternoon. The government brought the case to the appeals court under a rarely used procedural rule that allows for appellate review within 96 hours when a substantial question has been raised in a lower court trial.

The issue will be heard by a three-judge appeals panel that presumably will decide first whether the question about the regulation had been raised during a trial, and second, whether Murphy was correct in his interpretation of what he sees as the regulation's defects.

The dispute arose Thursday when the government attempted to introduce the City Council regulation into evidence in its prosecution of three District residents, each of whom is charged with one count of possession of phencyclidine (PCP). The charge is misdemeanor punishable by one year in prison and a $1,000 fine.

The regulation amended D.C. health regulations so that the five drugs would be included in the dangerous Drug Act of the D.C. Code, according to assistant corporation counsel William Robinson.

Defense attorneys objected to the government's action. They contended that the regulation was defective because it did not state specifically why the Council had found the drugs to be dangerous.

According to the code, a substance can be classified as a dangerous drug if the Council "shall find and declare by rule or regulation" that the substance is found to be "habit-forming, excessively stimulating, or to have a dangerous toxic or hypnotic or somnifacient" effect on humans or animals.

The Council, however, said in the regulation only that the five substances were "dangerous drugs within the meaning" of the code's Dangerous Drug Act, which Murphy found did not meet the requirements of specificity required by the code.

"The court therefore finds (the regulation) fatally defective," Murphy said during a hearing yesterday. He noted that it was his feeling that the regulation, as passed by the city council, did not give the public adequate notice to know that possession of the substances had been declared illegal.

During the hearing yesterday, Murphy said he found ambiguities in portions of the record of Council action that led to the disputed regulation, and contradictions as to whether the five substances contained all or one of the harmless qualities described in the code.

Murphy rejected government arguments that there was no requirement the Council declare which of the dangerous properties the five drugs had before they could validly be made a part of the Dangerous Drug Act.

Defense attorney Christopher Hoge argued Congress clearly intended in the D.C. Code that the Council declare, within the regulation, what the drugs, dangerous properties are.

Murphy apparently agreed with defense arguments saying Congress had intended that when a substance considered legal suddenly becomes illegal, as was the case with the five drugs, "the public has to know why."

Specifically, if the appeals court decides that the questiowas raised before the trial - which the government contends - the appellate judges could delay their decision on the validity of the regulation beyond the 96 hours set down in the court rules. Murphy ruled that the trial had started at the time the government attempted in introduce the regulation into evidence.