A controversial ruling on the obligation of producers of natural gas to assure uninterrupted deliveries to interstate pipelines was left in force yesterday because the Supreme Court divided evenly -- 4 to 4 -- over what to do about it.

Justice Potter Stewart held the swing vote but took no part in the consideration of two related cases involving the Federal Energy Regulatory Commission (FERC), the Consumer Energy Council of America, and Shell Oil Co. and other producers.

The court's one-sentence, unsigned opinion said only that the judgment of the 5th U.S. Circuit Court of Appeals "is affirmed by an equally divided court." As is customary, there was no explanation for Stewart's abstention.

Frequently, when the court has taken up a case involving a major oil company, however, Stewart and Justice Lewis F. Powell Jr. haven't voted on whether to hear it, have left the bench during oral argument and haven't voted on final action.

A year ago, trying to find out why, reporter Jim Mann, then of the Baltimore Sun, put the question to Stewart.

"I haven't said why, and I don't intend to," the Justice said. Why wouldn't Stewart say why? Mann asked. "Because it's the custom here not to," Stewart replied.

For Powell, a spokesman said he "feels it is improper" to comment. The other justices always are similarly adamant in refusing to explain abstentions.

Mann wrote that Stewart and Powell may have "represented one or more of the oil companies while they were in private practice. Or, more likely, it may be that they own shares of stock in one or more of the oil companies."

Next July 1, for the first time, the justices, along with numerous government officials, will be required by a broad new disclosure law to reveal their financial interests.

Yesterday's decision orginated in a 1976 FERC regulation that producers must "observe the standard of a prudent operator to develop and maintain deliverability" of gas from reserves that they have dedicated to interstate commerce.

The agency termed the ruling "erroneus" and warned that it "may seriously impair" its "ability to carry out its responsibilites under the... act." Under that law, which was displaced last year, many producers had "substantial incentives to cease or reduce their deliveries" arising from the then-existing differential between rates for intrastate and interstate gas, the FERC said.

The consumer council, which also had sued the FERC, argued that even the "prudent operator" standard was too weak to assure that dedicated interstate gas actually would be delivered.