WASHINGTON (AP) _ An appeals court let stand Thursday a federally required phase-in of air bags or automatic safety belts in cars and suggested that safety belt use laws in 26 states are unlikely to affect implementation of the federal requirement.

The 2-1 decision by a panel of the U.S. Court of Appeals fell short of what both the insurance industry, which filed suit to have the federal regulation overturned, or the Transportation Department had wanted.

Insurance groups have favored the phase-in of air bags or passive restraints, but they objected to the department linking the phase-in with the passage of state automobile safety belt use laws.

The suit contened that the department acted illegally when it said the federal requirement for air bags or passive belts would be rescinded if states with two-thirds of the population pass laws by 1989 requiring motorists to use belts.

But the appeals court refused to rule on the merits of the suit, saying that since the federal government will not decide on whether the rule is to be rescinded until 1989, the matter ''is not ripe for review'' at this time.

Twenty-six states have passed auto safety belt use law since the federal regulation was announced in 1984. But the court concluded that these laws are unlikely to be a threat to the federal regulation, as the suit had argued.

''None of these laws ... apparently complies with the secretary's specific requirements'' for belt use and ''it appears singularly unlikely that the passive restraint (standard) will be rescinded by 1989,'' the court said.

That language prompted insurance groups, including some of the plaintiffs in the case, to declare that even though no action was taken on the merits of the case, the ruling boosts the likelihood that the federal requirement for passive restraints will stay on its present course.

''We think it's a very significant gain for automotive safety,'' said James F. Fitzpatrick, an attorney for State Farm Mutual Auto Insurance Co., one of the plaintiffs in the case.

Brian O'Neill, president of the Insurance Institute for Highway Safety, said the court ruling ''means the (federal) requirement for automatic restraints can be expected to stand as written.''

''We'll be seeing more and more cars with air bags or automatic seat belts. That's welcome news,'' O'Neill said in a statement.

Transportation Secretary Elizabeth Dole issued a statement saying she was ''pleased with the court's decision,'' but spokesmen said the department would have no further comment on the ruling because it is still under review.

Mrs. Dole in 1984 ordered air bags or automatic seat belts in new cars beginning with some 1987 models.

Under the rule, 10 percent of all new 1987 model cars would have to have a passive passenger restraint system for front-seat occupants. The number of cars required to have the devices is to increase to 25 percent on 1988 models, 40 percent on 1989 models and to all 1990 model cars.