A rear-seat passenger restrained only by a lap belt is exposed to greater risk of disabling or perhaps fatal injury during a car crash than a passenger wearing a lap-and-shoulder belt, a privately commissioned study says.

"Rear-seat occupants wearing only lap belts are exposed to substantially greater impact forces in the abdominal and lower spinal column area than occupants wearing lap-shoulder belts," the study said. "This greatly increases their risk of sustaining severe injuries, including disabling or fatal injuries to the spinal cord and abdominal organs, in such crashes."The study was released by the Institute for Injury Reduction at a hearing this week of the House government activities and transportation subcommittee. The institute, based in Dunkirk, Md., was founded this year by attorneys involved in product-injury litigation.

The study was prepared for the institute by Automotive Safety Testing Inc. of East Liberty, Ohio, which used instrumented dummies belted into the rear-seat assembly of a 1987 Honda Accord LX and subjected to simulated crashes.

A simulated crash at 27 mph showed an adult-size dummy pressing against the outside part of a lap belt at 2,350 pounds per square inch, 338 percent higher than the pressure recorded when a lap-and-shoulder restraint was used.

The National Highway Traffic Safety Administration said the study showed a lack of responsibility toward public safety.

"In our opinion, it is irresponsible for anybody to discourage the wearing of any kind of safety belt," said Ron De Fore, NHTSA's director of public and consumer affairs. "The Department of Transportation has for years studied hundreds of thousands of fatal crashes, and our data shows that you have a nine times greater chance of being killed in the rear seat without a belt as opposed to wearing the belt, even if it is a lap-only belt."

De Fore accused the institute of special pleading. "The people who presented the testimony . . . have an underlying motive by their own admission - they represent plaintiff attorneys who have clients that are among the few that have been injured by the lap belt," De Fore said.

The use of lap belts alone in rear seat became a subject of contention among auto regulators with the 1986 release of a National Transportation Safety Board study that concluded the belts might cause serious injury rather than protecting the passenger.

The Highway Traffic Safety Administration has said the 1986 study was based on insufficient data.