The federal government plans to require automakers to install rear-seat shoulder belts in all new cars beginning with the 1990 model year.

The announcement Wednesday by the Transportation Department came just hours after a private research group, the Institute for Injury Reduction, publicly criticized the department for not requiring shoulder restraints in rear seats.Critics for several years have questioned the degree of safety provided by belts that fit only across the lap. Most cars have this kind of belt in the rear seat.

Benjamin Kelley, president of the Institute, said motorists are faced with "a pick your poison choice" because in some types of accidents the lap belts provide added protection, while in others they contribute to the severity of injuries.

The Institute was formed by trial lawyers involved in auto accident cases.

A National Transportation Safety Board study two years ago said that in some cases the lap-only belt made some injuries more severe because they caused the body to violently jackknife upon impact.

Diane Steed, administrator of the National Highway Traffic Safety Administration, said in a statement that the lap and shoulder belt combination in rear seats "will be an important safety addition, provided they are used." Her agency is part of the Transportation Department.

Steed also urged motorists whose cars are equipped only with lap belts in the rear to use them. She said they are "effective in protecting passengers in a crash, especially in preventing the injuries and fatalities that occur when people are ejected" from their vehicle.

The proposed requirement for shoulder belts in rear seats is not likely to become final until next year. Initially, it would apply only to new cars and does not address the question of retrofitting the tens of millions of automobiles now on the road that have lap-only belts for rear-seat passengers.

The rule would apply to small trucks, convertibles, vans and utility vehicles starting with the 1991 model year.

At a news conference Wednesday in which it criticized the lack of a federal requirement for rear-seat shoulder belts, the Institute for Injury Reduction also charged that the design of many shoulder-restraint seat belts is dangerously flawed.

The Institute said the "window shade" belt design used in an estimated 120 million cars, including virtually all U.S. models built since the mid-1970s, should be altered because it permits the belts to be worn too loosely.

The design allows the shoulder restraints to loosen when a car occupant shifts forward or sideways. Automakers say this is desireable because it makes the belts more comfortable, thus encourages use.