A federal appeals court on Tuesday threw out an agreement that Georgia reached with the Army Corps of Engineers for water rights to a major federal reservoir outside Atlanta, handing Alabama and Florida a major victory in the states' long-standing water wars.

The ruling comes amid tense negotiations among the states' governors over water sharing during a record drought.

The 2003 agreement with the Corps would give Georgia about a quarter of Lake Lanier's capacity over the coming decades and is the foundation of Georgia's long-term plans for supplying drinking water to the rapidly growing Atlanta region.

Alabama and Florida challenged the pact, arguing that Georgia doesn't have any legal right to the federal reservoir, which was initially built for hydropower. The withdrawals would dry up river flows into their states that support smaller municipalities, power plants, commercial fisheries and industrial users like paper mills.

A district court earlier ruled in Georgia's favor, but the U.S. Court of Appeals in Washington overturned that decision Tuesday, saying that the agreement constituted a major operational change at the reservoir that requires congressional approval.

"This is the most consequential legal ruling in the 18-year history of the water war, and one of the most important in the history of the State of Alabama," said Alabama Gov. Bob Riley. "It establishes that the decades-old practice of Atlanta taking more and more water from the federal reservoirs in the Coosa and Chattahoochee rivers without any legal authority to do so will not stand."

Although the ruling raises questions about Georgia's rights to the water it already is using in Lake Lanier, Bert Brantley, a spokesman for Georgia Gov. Sonny Perdue, said it would not harm the state's negotiating position in the water-sharing talks.

"Our goal and our focus has always been on reaching an agreement with our neighbors and to work this out at the negotiating table rather than in a courtroom," Brantley said. "There's a good bit of momentum built up right now for making progress."

He said the state had not ruled out appealing the case to the Supreme Court.

Florida Gov. Charlie Crist's office and the state Department of Environmental Protection did not return phone messages seeking comment.