WASHINGTON — The U.S. Supreme Court said Monday it will let stand a lower court ruling that threatens to unravel Georgia's long-term water plans for the Atlanta region, giving Florida and Alabama a pivotal victory in the states' long-running water wars.

The court's decision raises fundamental questions about Georgia's rights to Lake Lanier, a huge federal reservoir outside Atlanta that serves as the city's main water source. It could also play a key role in deciding related water rights disputes in lower courts.

The case involves a 2003 water-sharing agreement with the Army Corps of Engineers that would have allowed Georgia to take far more water from Lanier for its drinking supply over the coming decades. The deal would have allowed Georgia's withdrawals to jump from about 13 percent of the lake's capacity to about 22 percent.

Florida and Alabama objected, saying that the lake was initially built for hydropower and that providing water to Georgia is not an authorized use.