About 900,000 more air travelers would board planes at Dulles International Airport each year if they could catch their preferred flights there, a Council of Governments survey has concluded. Such growth would almost double the number of boardings at the airport.

About 200,000 more people would begin their trips at Baltimore-Washington International Airport, a rise of more than 20 percent, if service there matched demand, according to the study.

Gains at those two airports would decrease use of National Airport, which would lose about 1.1 million boarding passengers, the study said. That is about 16 percent of its current total.

The report, based on 28,000 questionnaires completed by people who departed from area airports in July 1981 and March 1982, tended to support contentions by promoters of Dulles and BWI that the two airports are underserved.

Airline spokesmen contacted declined immediate comment on the study. But carriers generally have argued that their service is centered on National, located only a few minutes from downtown, because they believe that is where the demand is. Dulles and BWI are both located well outside the Capital Beltway.

United Airlines spokesman Charles Novak, stating that United's typical short-haul passenger in the Washington market is a businessman heading to or from downtown, said: "There's no reason to drive beyond National."

The results come at a time when traffic at Dulles has expanded, apparently due at least in part to restrictions placed on National after the nation's air traffic controllers went on strike last year. No limits were imposed at Dulles.

The Federal Aviation Administration, which owns and operates both airports, reports that passengers beginning or ending their trips at Dulles increased by 4.5 percent to 2.46 million in the year ending July 31. At National, passengers decreased by 7.3 percent to 13.39 million in the same period.

What will happen as strike-related limits are lifted remains unclear. Last month, the FAA discontinued the strike restrictions at National, though traffic there remains below prestrike levels because limits are still in force at other big airports used by flights in and out of National.

This week the FAA terminated strike quotas in the 140,000 square miles of air space directed by the Washington en route control center at Leesburg, Va. This is expected to have only marginal effect for the present, because only flights taking off and landing entirely within the center's zone could return to prestrike levels. Any flights entering or leaving neighboring centers' areas would be affected by continuing restrictions there.

Nationwide, air traffic is operating at about 85 percent of levels before the strike, according to the FAA. Full capacity will be restored in December 1983, the agency has said.