Warning that "an appalling and continuing high level of defects in the manufacture and design of seat belts" is causing hundreds of highway deaths and injuries each year, a consumer group has called on the top federal auto safety regulator to evaluate carefully the merits of all occupant restraint systems.

In a letter to National Highway Traffic Safety Administration head Joan Claybrook, the Center for Auto Safety said its own six-month survey of the performance characteristics of seat-belt systems in domestic and foreign cars has led to the conclusion that seat belts are "the single most defect-prone item of automobile equipment foisted onto an unsuspecting motoring public."

Besides being dangerous because they are in some way defective, existing seat belt systems are "too inconvenient, unreliable or uncomfortable to use," center executive director Clarence Ditlow claimed in the letter.

Responding to NHTSA's own figures that only 14 percent of the driving public actually uses seat belts, the center's survey reveals 'the primary reason given for not wearing seat belts is that they are uncomfortable to wear and inconvenient or confusing to use.

"Compounding the inconvenience and discomfort factors in inducing poor use (or seat belts) are frequent failures or defects," the center claims. "A belt that comes unbuckled or pulls loose in a crash provides as little protection as a belt that isn't worn. A belt that is badly frayed or jammed from a defective retraction mechanism will not be worn."

Slightly more than 1,200 complaints from consumers, including reports of 92 accidents involving 97 injuries and three deaths, provided the basis for the survey by the CAS. But a full 1,000 of the complaints, the CAS said, "were made about durability related problems and defects."

Foreign car belt system complaints seemed to center on broken retractors, buckles and anchors. There were an unusually high number of complaints from Ford owners that their belts were too short. Chrysler and AMC owners beefed mostly about broken retractors, while the highest number of General Motors complaints involved broken anchors and buckles.

"Serious questions are raised by American motorists about the useful life of seat-belt system," the CAS study contends. "Consumers report a plethora of durability problems: straps wear out, buckles break, seat belt guides and covers wear out."

"One consumer noted that the anchorage of one belt was so insecure that it was pulled entirely out of the floor by her 4-year-old child," the group said. The report contends that continued failures of belts encourages non-use by those who experience trouble.

Further, the group claims, existing Federal Motor Vehicle Safety Standards for seat belts -- which set requirements for abrasion and light resistance -- "have little, if any, relation to real world durability problems."

Concluding that present seat belt systems are "woefully inadequate in protecting consumers in vehicle crashes," the CAS study calls on NHTSA to investigate and recall all defective seat belts, set new and tougher seat-belt performance standards, and work with the auto industry to improve the confort and convenience of belts.

In an interview, Claybrook responded by pointing out that her agency already is working on a proposed standard for seat belt "comfort and convenience."

"Clearly, if belts are more comfortable and convenient, and people become more knowledgable about their value, we would increase their use," Claybrook said. "But even in Canada, where there is a usage requirement, the maximum use is about 50 percent."

In a recent government-sponsored test, Claybrook said, 30 different 1979 models -- 19 domestic and 11 foreign -- were tested by 114 models of different size and gender for seat-belt comfort and convenience.

The best cars, which still gave some sort of problem to 35 percent of the testers, were the Ford Fairmont, Mazda, Subaru, Cadillac DeVille, Mercedes, Toyota Corolla, Datsun and Volvo.

The worst cars, she said, with complaints by about 75 percent of the testers, were the AMC Concord, AMC Pacer and the Oldsmobile Cutlass.

As an example of the value of comfort and convenience, Claybrook said, the Volvo, Subaru and Mercedes, cited as more comfortable, also are among the cars in which the seatbelts are most used.