United Air Lines and three European air carriers will join forces to create an international computer reservation system, executives of the airlines said yesterday.

The move appeared to be a major step for United in the increasingly heated battle for revenue from the profitable business of computer reservation systems.

United's Covia subsidiary, which runs the airline's Apollo reservation system, will develop a new international system jointly with British Airways, KLM Royal Dutch Airlines and Swissair that they say will handle an estimated 75 million travel reservations annually by 1990.

The system will link the computer reservation systems operated by the four airlines by late 1988 and operate using a new IBM-based central system by mid-1989.

The airlines will invest about $120 million in the venture. "The airlines joining us and the agents who use the system can anticipate real productivity benefits throughout the 1990s," said John Seeman, executive vice president for marketing and planning for United. United officials said they hope other European airlines will join the consortium.

Last month Air France, Lufthansa, Iberia and Scandinavian Airlines System formed a consortium called Amadeus to develop a joint computer reservation system, expected to begin service in 1989. Like the venture announced by United and the European carriers yesterday, it will offer travel agents and others who use the system information about hotels, airlines and car rental reservations.

United spokesman Rob Doughty said that the United group has several advantages, including the fact that it is based on an IBM system. However, the Amadeus group controls 60 percent of the scheduled travel from one European country to another.

One advantage of the consortium for American travelers will be that it will allow them to make reservations on European railroads, Doughty said.