WASHINGTON (AP) _ A private research group on Wednesday charged that the seat belts in a vast majority of cars are poorly designed and don't provide full protection unless motorists adjust them to fit tighter.

The National Highway Traffice Safety Administration, meanwhile, announced it will require shoulder harnesses in the rear seats of all cars, effective in the 1990 model year starting next September.

Both lap and shoulder harnesses are now required for front-seat passengers, but only lap beats are legally necessary in the rear. Many automakers have provided shoulder belts for all seats anyway, especially in the wake of adverse court rulings on injury suits.

On the separate design issue, the Institute for Injury Reduction assailed the ''window shade'' feature allows most belts to loosen when a car occupant slowly shifts forward or sideways.

This design, used in an estimated 120 million cars, including virtually all U.S. car models built since the mid-1970s, often keeps the belt resting some distance from the body, the institute said.

Unless the shoulder restraint is manually tightened - with a slight tug forward on the belt - its effectiveness in a crash may be reduced significantly, Benjamin Kelley, president of the institute, told a news conference.

The institute, which was formed by a group of trial lawyers involved in automobile accident litigation, called on the NHTSA to eliminate the window shade design - or short of that begin a campaign to warn motorists about the alleged design flaw.

But U.S. automakers contend there is no flaw at all - and the highway safety agency agrees.

''We have no evidence that window-shade (belt designs) are defective or present a safety problem,'' NHTSA Administrator Diane Steed said in a statement responding to the institute's charges.

Auto manufacturers adopted the belt design in the mid-1970s and they defend its use because, they said, it helps get people to accept the belts. Motorists previously had objected to tight-fitting belts as being too uncomfortable.

''The concept was to make the belts more user friendly so we could get the percentages of (belt) users up,'' said Bill Peacock, a spokesman for Ford Motor Co., who said the tension reliever - or window shade - system cost Ford extra money.

John Hartnett, a spokesman for General Motors Corp., called the seat belt design ''an effective and comfortable seat belt system. It encourages belt use.''

Industry spokesmen as well as NHTSA officials acknowleged that if the belts are to provide their best protection they should be worn snugly. But they suggest the tradeoff in having belts that may be slightly loosened is that fewer people will reject the belts altogether.

The window shade system is designed on a pendulum principal that locks the belt tightly when the car decelerates suddenly, as in a quick stop or an accident, while allowing the user to move normally at other times.

Kelley argued, however, that the belts sometimes stay loose after the user shifts about.

''The (belt) slackness, which people often are not even aware of, can result in horrendous injuries,'' he said, while claiming that automakers have the technology available to develop belts that do not loosen and still meet motorist acceptance.

Kelley cited 1982 tests by NHTSA which compared the expected injury in a 30 mph crash when belts have various amounts of slack.

With 1 inch of belt slack, a test dummy sustained a head injury level of about 50 percent greater than if there were no slack at all; at 2 inches of slack the injury level was nearly twice as severe; and with a full extension, or nearly 17 inches, it was about 4 1/2 times as great.

NHTSA officials acknowledged the test results, but noted that in all the tests except for the one conducted with a full extension of slack, the head injury levels remained below what would be considered life threatening. With up to a 2-inch slack there was ''no signficant degradation of safety,'' said Barry Felrice, associate adminstrator for rulemaking.

The lawyer group also reiterated its complaint that only lap belts are in rear seats of an estimated 150 million cars. They suggested the belts may in some accidents actually cause serious injury because of the jack-knifing of passengers without shoulder belts.

Within hours, the NHTSA announced the new requirement for shoulder harnesses in the rear.

The National Transportation Safety Board several years ago raised a firestorm when it came to a similar conclusion after studying a number of accidents. Since then many manufacturers have put shoulder-harness belt systems in the rear seats.

According to Peacock, about one-third of the 1989 Ford cars have rear-seat shoulder belts. Hartnett said all 1989 GM model cars have shoulder belts in both front and rear seats. Many imports have for some time had such belts available for passengers in the rear.

Nevertheless, complained Kelley, many older cars continue to have lap-only belts in rear seats, exposing passengers to the danger of sustaining serious abdominal or head injuries.

NHTSA's Steed criticized Kelley and the lawyer group for suggesting that in some cases passengers might be better off not wearing a belt.

''No matter where you are sitting or what type of belt you have available, wearing a belt is better than not wearing one,'' she said.