The airports board approved the creation yesterday of a downtown airport services complex where passengers could buy some airline tickets, receive boarding passes, check baggage and browse in a duty-free shop while waiting to board buses to Dulles International and National airports.

"It's almost like building an additional terminal," said Richard Griesbach, the airports' business manager, after the board of the Metropolitan Washington Airports Authority endorsed the staff proposal.

The board's approval means the airport staff can start drawing up contracts and seeking the necessary city approvals, and possibly open the facility by April, Griesbach said.

The new complex would relieve some of the increasing congestion in Dulles' main terminal, particularly in the duty-free shop that has been overwhelmed recently by rising numbers of Japanese tourists, airport officials said.

In their report, airport managers proposed establishing the complex in the Investment Building, 1511 K St. NW, next to the Capital Hilton Hotel, which serves as the downtown departure point for buses to National and Dulles and Baltimore-Washington International Airport.

The new facility could accommodate ticketing for up to three airlines, according to the proposal. Because many airlines operate ticket offices on K Street NW, within a block or two of the proposed new facility, the airport planners said space for three airlines should be adequate.

Baggage check-in services could be provided for all airlines serving all three airports, if the airlines agreed, Griesbach said.

BWI has been invited to move its bus operations to the new complex and is "very interested in sharing the facility," said spokeswoman Mary Hope.

Greyhound Leisure Services Inc., which manages the Dulles duty-free shop, has offered to lease 12,265 square feet of office space for the downtown terminal, and pay for about $1 million in construction and equipment costs. The airports board voted yesterday to extend Greyhound's concession contract through March 1993, to allow the company to recover the costs of its investment.

The new facility would involve no cost to the authority, which plans to finance $700 million in improvements at National and Dulles. The downtown shop also is expected to generate increased revenue for the airports, which receive a percentage of all concession sales, Griesbach said.

"It's a fantastic opportunity," said Clyde Bingman, the authority's manager of commercial programs.

The need for a downtown terminal arose out of the extraordinary growth at Dulles over the last year and the Hilton's decision to move the bus operations out of the hotel, airport business managers said.

Dulles, the fastest-growing airport in the country in recent years, saw its duty-free shop sales climb 425 percent in the first six months of this year, compared to the same period last year. Much of the sales growth is attributed to the addition last year of nonstop service to Tokyo by All Nippon Airways.

Although the Dulles duty-free shop was recently expanded, it "is still unable to accommodate" the crowds, causing departure delays, particularly for All Nippon Airways, the staff report said. The delays happen because of the time required to order merchandise from the duty-free warehouse and deliver the purchases to passengers as they board their flights.

Airport managers began thinking about a downtown shop about a year ago because of the impossibility of further expanding the Dulles shop within the cramped and crowded main terminal.

At the same time, the Capital Hilton asked the airports to move their bus operations because of hotel renovations, Bingman said. Since hotel construction began, the Washington Flyer and BWI bus services have been "operating on the curb," with no seats, shelter or ticket counter, he said.

All Nippon Airways has offered to sublease space in the new facility for ticket sales, and one unidentified airline has "expressed interest" in operating there, Bingman said.

In addition, all Japanese tour groups visiting Washington on the airline will have a scheduled stop at the downtown duty-free shop, according to terms of a joint venture of the airline and Greyhound Leisure, the airport staff report said.

The arrangement "would benefit everyone," the report said.