NEW YORK, AUG. 31 -- Eastern Airlines announced today it will cut its lowest discount fares, in a move that threatens to derail attempts by rival carriers to raise those fares during the post-Labor Day travel lull.

Eastern, a unit of Texas Air Corp., the nation's biggest airline group, said that from Sept. 9 through Dec. 15 it will cut an average of $20 one way from the deep discount rates commonly known as "maxsavers," the industry's cheapest fares.

Delta Air Lines and Northwest Airlines -- which last week announced they were raising maxsaver rates -- said they would match the fare cuts in markets where they competed with Eastern.

An American spokesman at the carrier's Dallas headquarters indicated it was studying the fare cuts, while Chicago-based United Airlines did not return telephone calls seeking comment.

United, American, Delta and Northwest all announced last week they were raising maxsaver rates by $10 to $40 each way beginning with tickets purchased on or after Sept. 8.

In addition, the airlines tightened advance purchase requirements on their maxsavers from the previous seven days to as much as 30 days.

In raising their fares, the airlines had pointed to rising fuel costs and strong summer passenger traffic, even though airlines normally reduce fares at this time of year to cope with the typical post-Labor Day travel lull.

But analysts said that as with other major fare restructurings, the bellwether would be Texas Air, which controls about 20 percent of the U.S. air travel market and generally is regarded as the industry's pricing leader.

"The fact that one carrier like Texas Air decides not to go along with the price increase makes it difficult for the others to continue with their plan," said Robert Decker, who follows the airlines for Chicago investment firm Duff & Phelps.