WASHINGTON — Florida and Alabama are asking a federal court to stop the Army Corps of Engineers from supplying water to Georgia from Lake Lanier, Atlanta’s main water source.

The states argue the withdrawals are illegal without congressional approval. They said they would file a motion in U.S. District Court in Jacksonville Friday to invalidate the corps’ operations on the federal reservoir.

Florida and Alabama have long argued that the corps is misusing Lanier at the expense of their interests downstream. The latest motion comes two weeks after the U.S. Supreme Court let stand a lower-court ruling that said the corps needed congressional approval before reaching a 2003 agreement to give Georgia even more water from Lanier in the coming decades.

Emboldened by the decision, Florida and Alabama say that rationale should also apply to Georgia’s current withdrawals.

Their motion also comes after the corps issued an opinion this week saying that it has legal authority to supply metro Atlanta’s drinking water from Lanier.

Corps attorney Earl Stockdale’s opinion said that setting aside about 12 percent of the lake for drinking water would have only minor effects on the project’s goals to produce hydropower, control floods and float barges.

The reservoir is at the heart of the states’ two-decade-old water wars in which Georgia maintains it needs more water to serve a rapidly growing population, while Florida and Alabama argue that the state’s withdrawals harm their downstream interests.

Judge Paul A. Magnuson, who is overseeing combined lawsuits involving the Apalachicola-Chattahoochee-Flint river basin, has said that he will take up the question of Lanier’s authorized uses first.