The Metropolitan Washington Airports Authority approved plans yesterday to expand one of the temporary midfield terminals at Dulles International Airport to accommodate growing numbers of flights.

United Airlines and Trans World Airlines plan to add nine gates to United's 11-gate terminal, a temporary structure built of trailers on Dulles' jet ramp. Passengers are ferried to the midfield facility from the main terminal by mobile lounges.

Construction already is under way to add six gates to Continental and American airlines' 10-gate midfield terminal that, like United's, was built as a temporary solution to the gate shortage that has taken place at Dulles as its business has soared.

The new authority, which assumed control of Dulles and National on June 7, is considering plans to build two to six permanent midfield terminals linked by an underground people mover to the main terminal, at a cost of at least $35 million.

The board of directors will meet July 11 to discuss the proposal, which has caused concern among Continental officials, who have estimated the ultimate costs at $185 million and argue that less expensive alternatives should be examined.

United and TWA plan to complete the $14 million expansion of the temporary terminal by May, said John Philip, a United official. United and Air Wisconsin, operating as a feeder service under the name "United Express," operate 113 departures a day at Dulles, and expect that number to exceed 155 by next summer, he said.

In the 12 months that ended May 31, commercial flights at Dulles increased by 80 percent and passengers increased by 73 percent, according to the FAA.