The California Supreme  Court upheld a state law on Monday allowing illegal immigrants who attend high school in California for three years and graduate to pay lower in-state tuition rates at state colleges and universities.

The unanimous ruling reverses a lower-court decision siding with opponents of the law who said it unfairly favors illegal immigrants over U.S. citizens who live outside California and are charged much higher out-of-state tuition rates.

The measure, passed by the state legislature in 2001, was challenged in court on behalf of students who are U.S. citizens and claimed they were illegally denied the in-state tuition break carved out for undocumented students.

Nine other states have adopted similar tuition laws, and legal challenges are pending in two of them -- Nebraska and Texas, lawyers for both sides in the California case said.

Kris Kobach, attorney for the Immigration Reform Law Institute, which brought the California suit, said Monday's decision was "flawed in numerous ways" and vowed to appeal to the U.S. Supreme Court.

Kobach, whose group estimates more than 25,000 illegal immigrants pay in-state tuition each year under California's statute, said there was a good chance his side would prevail if the nation's high court agreed to hear the case.