The Justice Department yesterday recommended against requiring major airlines to sell their highly successful computer reservation systems as a means of stimulating competition in flight reservations.

About two-thirds of all airline tickets are sold by travel agents and about four-fifths of all travel agents book their flights using either United Airline's Apollo or American's Sabre reservation systems.

Competitors had complained that the systems can give American and United an unfair competitive advantage, but Justice said there does not appear to be any need for major changes in the rules governing such systems.

Justice officials said the cost to the two big airlines of selling their system would be high and that competition from a potential non-airline-owned competitor could act as a corrective to abuses.

The now-defunct Civil Aeronautics Board adopted regulations late last year to address what was seen as the major abuse of reservation systems. That regulation is now enforced by the Transporation Department.

Airlines without their own computerized reservations networks have charged that United and American are able to dominate the market in part because of the information they have about everybody else's schedules and fares. Almost all airlines provide their schedules and fares to Apollo and Sabre, fearing travel agents may ignore flights not listed in the computer system.

Travel agents also use the airline reservation systems to book flights, hotel rooms and rental cars, to issue boarding passes and to record mileage for travelers who belong to various frequent flyer programs.

The biggest problem with the systems found in the CAB investigation -- favoritism toward the airline owning the reservation system -- was forbidden by the new rules. The systems now are not supposed to be biased against any airline.

Despite that corrective, other airlines have said that already wealthy United's and American's advantage continues because of the fees they charge other airlines to process ticket transactions.

Justice said the bottom line is that "the dominant CRS vendors apparently continue to possess market power." However, it said, while the exercise of that power could harm competition, "It is unclear that any such harm is in fact occurring."

Justice said "it is uncertain" whether CRS vendors are charging unfair prices to rival airlines, and is too early to determine the effect of the CAB's rules.

The Justice study suggests that, although the most obvious bias in the displays has been corrected, there may be other ways to influence travel agents in their selection of airlines.

One way of selecting which flight to display first is the length of the flight. American, in its Sabre system, "has routinely entered elapsed flying times that match the lowest elapsed time of other competing carriers, regardless of the true elapsed time," Justice said. It said that American contended that this practice was started in response to similar conduct by some of its competitors.

A United spokesman declined comment. An American spokesman said, "I think we would certainly support" the Justice conclusion that divestiture is not indicated.