The U.S. Court of Appeals yesterday ordered the Reagan administration to reinstate a rule it revoked last year that would have required air bags or automatically closing seat belts in all cars.

In a three-page order, a three-judge panel of the federal appeals court here said the National Highway Traffic Safety Administration must require such "passive restraints" in all cars by Sept. 1, 1983--unless the agency can prove within the next 45 days that more time is needed for automobile manufacturers to comply with the rule.

The court decision represented another setback in the Reagan administration's efforts to repeal what has been considered the most important and controversial of federal traffic safety regulations ever issued by the government.

NHTSA's administrator Raymond A. Peck repealed the rule last October over the objection of most of his staff, thereby ending the government's 12-year-effort to equip cars with air bags or automatically closing seat belts. At the time, Peck argued that the rule would not provide a significant increase in passenger safety because the companies had abandoned air bag development and the automatic seat belts they planned to use instead would not represent a major improvement over manual belts.

Two months ago, the U.S. Court of Appeals found Peck's decision to be "arbitrary and unlawful," but decided to give NHTSA additional time to review its decision.

Yesterday, the court found that NHTSA's timetable for review was unacceptable. The court ordered NHTSA to put the rule into effect for all new cars manufactured after Sept. 1, l983, the beginning of the 1984 model year.

Government lawyers said NHTSA would undoubtedly ask the U.S. Supreme Court to review the decision and seek a stay of the ruling until the high court acted.

Sources said NHTSA would ask the appeals court to delay the September 1983 effective date, arguing that automobile manufacturers could not possibly adjust their assembly lines in time.

Peck issued a statement defending his decision, saying it was rescinded because "it no longer met the statutory criteria which requires that all standards be 'reasonable,' 'practicable,' 'appropriate,' and meet 'the need for highway safety.' " Peck will continue his publicity campaign to persuade more motorists to buckle up voluntarily, an effort he says he believes will be more effective in saving lives.

If upheld, the appeals court decision would give the automobile industry one more year to install passive restraints in large and mid-size cars than the original rule would have provided. Under the rule issued by the Carter administration, air bags or automatically closing seat belts would have been required in all new large and mid-size cars as of September 1982 and in all small cars by September 1983.

But automobile manufacturers said yesterday that it may be equally impossible to meet the new compliance date. "There is absolutely no way that Chrysler Corp. can meet the federal appeals court requirement to install automatic passive restraints in all of its new cars by the fall of 1983," the nation's third-largest auto manufacturer said in a statement yesterday.

Roger E. Maugh, automotive safety director for Ford Motor Co., said that the "effect of the court order is to require us to divert scarce resources in an effort to meet a standard" that the company still believes will be ineffective in improving highway safety.

Consumer groups and insurance companies hailed the decision. "We are extremely gratified . . . that the court properly recognized that the lives of thousands of American motorists hung in the balance," with the passive-restraint rule, said Lowell R. Beck, president of the National Association of Independent Insurers, which represents more than 500 companies.

Attorneys for State Farm Mutual Automobile Insurance Co. said statements by NHTSA, Chrysler and Ford earlier in the regulatory proceedings showed that one year would be enough lead time for installation of automatic restraints.