An Environmental Protection Agency official has ruled that EPA doesn't have authority to penalize gasoline users who illegally put leaded gas in cars designed for unleaded fuel.

The ruling, handed down last week, raises questions of whether EPA can carry out one of its top-priority enforcement efforts as effectively as in the past. EPA judicial officer Ronald L. McCallum, on behalf of administrator Anne M. Gorsuch, said EPA lacked congressional authority to conduct enforcement hearings and impose penalties on violators of the Clean Air Act fuel use sections. That authority lies in the federal courts, he ruled.

Prior to his finding, EPA took the position that the federal courts couldn't handle enforcement actions promptly because of the number and complexity of the cases. EPA held hearings to consider evidence of violations and then determined whether penalties were warranted. Between 1977 and 1981, some 2,000 cases were handled in this way.

The enforcement procedures were designed to prevent service station operators, auto fleet owners and other gasoline distributors from using cheaper leaded gasoline in cars designed to run only on unleaded fuel. Such fuel switching has been a major concern, since lead compounds in leaded gasoline can destroy the expensive catalytic converters used to control pollution on most U.S. cars built since the mid-1970s.

One of these cases prompted the change in policy ordered last week. It involved penalties of $315,000 assessed against Arlington Yellow Cab Inc., All State Messenger and Delivery Service Inc., Transportation Inc., and Murphy Bros., Inc. EPA charged that vehicles owned by these firms were illegally using leaded gas supplied by company pumps.

The companies charged that EPA exceeded congressional authority. Their case was denied by administrative law judge Gerald Harwood on June 15. But McCallum, a career EPA legal officer who handles such judicial appeals for the EPA administrator, overruled Harwood.

McCallum said that EPA can still enforce the Clean Air regulations with "reasonable effectiveness" by bargaining with alleged violators before cases are brought in federal court.

Benjamin R. Jackson, former EPA deputy assistant administrator for mobile source enforcement, expressed concern that enforcement might be slowed down. EPA has wanted a powerful deterrent to prevent illegal fuel switching, Jackson said, because of the tremendous environmental and economic damage that could be done if large numbers of motors began using leaded fuel illegally.