A federal judge has ruled that Georgia must stop withdrawing water within three years from a federal reservoir outside Atlanta that is the city's main water supply.

U.S. District Court Judge Paul Magnuson ruled Friday that Georgia must stop withdrawing water from Lake Lanier unless it can get permission from Congress to do so.

The judge acknowledged it would be impossible for the state to immediately stop using the lake because it is Atlanta's main water supply. So he is allowing three years for the state to seek proper authority from Congress.

The judge says Lake Lanier wasn't built for water supply and that Atlanta's withdrawals from the lake are illegal. The water from Lanier flows into the Flint River which eventually joins the Apalachicola River in Northwest Florida.

Florida, Georgia and Alabama have been in a legal and political battle over water rights since the early 1990s.