WASHINGTON — A federal appeals court on Friday blocked Kansas, Georgia and Alabama from requiring residents to prove they are U.S. citizens when registering to vote using a national form.

The 2-1 ruling is a victory for voting rights groups who said a U.S. election official illegally changed proof-of-citizenship requirements on the federal registration form at the behest of the three states.

People registering to vote in other states are only required to swear that they are citizens, not show documentary proof.

The three-judge panel of the U.S. Court of Appeals for the District of Columbia acted swiftly in the case, issuing a two-page, unsigned ruling just a day after hearing oral arguments. A federal judge in July had refused to block the requirement while the case is considered on the merits.

The League of Women Voters and civil rights groups argued that the requirements could lead to the “mass disenfranchisement” of thousands of potential voters — many of them poor, African-American and living in rural areas.