A three-judge panel of the 9th U.S. Circuit Court of Appeals found the law requiring proof of citizenship for voter registration -- passed in a 2004 referendum -- violates the National Voter Registration Act, Tucson's Arizona Daily Star reported Tuesday.
The panel included former U.S. Supreme Court Justice Sandra Day O'Connor, temporarily sitting on the circuit bench, the newspaper said.
O'Connor and Judge Sandra Ikuta found the initiative, Proposition 200, sets up an additional obstacle to voting while the federal voter registration act is intended to reduce "state-imposed obstacles" to registration. The finding held that the federal law provides protections against voter registration fraud.
The court found Arizona's requirement that voters produce photo ID at the polls does not violate the Constitution.
Arizona officials said they plan to appeal the ruling.