The US supreme court ruled on Monday that states cannot demand proof of citizenship when prospective voters register to vote.

The 7-2 ruling that struck down Arizona's voter-approved requirement that voters prove they are US citizens in order to use a federal registration form, could affect other states with similar laws, including Alabama, Georgia and Kansas. Twelve additional states are contemplating similar legislation.

States say the laws were designed to prevent voter fraud, but civil rights groups said the Arizona law was aimed at discouraging certain groups from voting, such as Native Americans, the elderly and minorities.

The case began seven years ago, when Arizona residents, civil rights groups and members of Native American tribes sued to challenge the state measure, Proposition 200, which they said discriminated against eligible voters.

Arizona, which shares a border with Mexico, has clashed repeatedly with the federal government over passing tough immigration laws.

Arizona officials say they should be able to pass laws to stop undocumented migrants and other non-citizens from getting on their voting rolls. The voter-approved law, Proposition 200, was part of a package that also denied some government benefits to undocumented migrants in addition to requiring Arizonans to show identification before voting.

The court was considering whether a 1993 federal law known as a the Motor Voter Act pre-empts Arizona's state law requirement that officials reject a federal application when it is submitted without proof of citizenship. Federal forms do not require proof of citizenship.

Justice Antonin Scalia, writing for the court's majority, found that federal law "precludes Arizona from requiring a federal form applicant to submit information beyond that required by the form itself".

Justices Clarence Thomas and Samuel Alito, two members of the court's conservative wing, dissented.

Arizona was appealing a decision of the ninth US circuit court of appeals, which said that the National Voter Registration Act of 1993, which doesn't require documentation to prove US citizenship, trumps Arizona's Proposition 200.

Arizona voters approved Proposition 200 in 2004, requiring more specific proof of citizenship. During the next three years, more than 30,000 people were turned away for failing to provide documentation, according to civil rights groups. About 20% of those blocked were Latino.

Jon Greenbaum, chief counsel for the Lawyers Committee for Civil Rights under the law, said the ruling would be welcomed particularly by people who do not carry around identification, including students, Native American Indians and the elderly.

Greenbaum, who is acting on behalf of the Inter Tribal Council of Arizona, said it first brought the lawsuit in 2006, said: "The federal form is going to continue to provide a uniform, simple way for people to register to vote."

The "motor voter" law, enacted in 1993 to expand voter registration, requires states to offer voter registration when a resident applies for a driver's licence or certain benefits. It requires states to allow would-be voters to fill out mail-in registration cards and swear they are citizens under penalty of perjury, but it doesn't require them to show proof.