More than 60 major criminal cases in Prince George's County, including five murder prosecutions, are in jeopardy of being dismissed under a recent speedy-trial ruling by Maryland's highest court, the county state's attorney said yesterday.

The ruling, which set a mandatory 120-day deadline for beginning criminal trials, also could have serious ramifications in Montgomery County, but prosecutors there refused to speculate on the number of cases involved.

Responding to the furor the decision has caused, particularly in Baltimore courts, state Attorney General Stephen Sachs announced he will ask the Maryland Court of Appeals to reconsider its ruling.

The decision made mandatory a rule of court procedure, previously regarded as merely a guideline, requiring that a trial must begin within 120 days of the time a defendant's attorney officially enters the case or the defendant appears before the court for appointment of an attorney.

If the rule is violated, the court asserted, the criminal charges must be dismissed unless there is "extraordinary cause" for the trial's postponement.

A Prince George's County Circuit judge refused this week to dismiss a murder charge against a man accused of slaying two Washington cab drivers, even though the 120-day rule had been violated. He ruled that "extraordinary cause" for delay existed because the case involved legal wrangling over the state's new death penalty law.

The defendant, Eugene Jerome Black, now has gone to the appeals court in an attempt to have the charge dismissed.

According to Prince George's State's Attorney Arthur A. Marshall, the appeals court left unclear in its June 25 decision what circumstances qualify as "extraordinary cause" for postponement. It is also unclear whether the ruling covers cases dating back to July 1, 1977, when the 120-rule became effective, or only cases that come up after the court's decision.