The Fairfax County Board of Supervisors yesterday voted unanimously to call for establishment of a foreign trade zone near Dulles International Airport, a move that they said should improve the facility's sagging fortunes and bring millions of dollars into the county.

If approved by the federal government, the decision would allow businesses in the zone to avoid paying import duties on the materials used in products made in the zone but exported. They would have to pay the duties on any products sold in the United States.

The 25-acre site to be located south of Herndon would be the second such trade zone to serve metropolitan Washington. A Prince George's County facility is scheduled for completion late this year.

Fairfax officials said a foreign trade zone could attract between $5 million and $7 million in business to the county in its first five years of operations, luring trading firms thst wish to simplify their customs review process.

"Our thrust is getting this done as soon as possible," said County Board Chairman John F. Herrity, a Republican. "This could encourge not only foreign trade but local trade, as well."

In the same meeting, Fairfax supervisors voted to send Transportation Secretary Drew Lewis a sternly worded message threatening a lawsuit if Lewis does not withdraw his decision to postpone the effective date of proposed limits on air traffic to Washington National Airport.

County officials said a Belgium-Based corporation called Gestinvest Limited has been chosen to manage the trade zone after a nationwide search turned up no other applicants. Construction of the facility would begin next spring.

A foreign trade zone is a secured area under U.S. Customs supervision which is designated as international territory for customs purposes. International traders may bring goods into such zone for manufacturing, warehousing, exhibiting, processing and assembly, with duties being paid only when the goods are removed from the zone for consumption in the domestic market.