It would take an act of Congress to get more drinking water out of Lake Lanier for metro Atlanta, a federal appellate court ruled Tuesday.

Alabama and Florida immediately declared a major victory in the 18-year, tri-state water war, with Alabama Gov. Bob Riley calling it "one of the most important" legal decisions in his state's history.

"The ruling invalidates the massive water grab that Georgia tried to pull off," Riley said in a statement.

The decision by the U.S. Court of Appeals for the District of Columbia Circuit comes at a critical juncture, with the three states rushing toward a Feb. 15 deadline to reach a long-term, water-sharing agreement.

Observers say it gives Alabama and Florida leverage in the negotiations and belies metro Atlanta's assumption that it can count on Lanier to continue fueling its growth. Water from Lanier, the largest federal reservoir on the Chattahoochee River, forms Georgia and Alabama's southern border and winds up in Gulf of Mexico.

Lanier is the main water source for more than three million metro Atlantans. But it also supports multiple downstream users, from a nuclear power plant near Dothan, Ala., to oystermen in Florida's Apalachicola Bay.

"The big loser here is metro Atlanta," said George William Sherk, an expert in water law at the Colorado School of Mines who once represented the city of LaGrange and Troup County in tri-state water matters. "The logical response for metro Atlanta right now is no new building permits unless the applicant can demonstrate a long-term water supply.