A federal appeals court has refused to lift a stay blocking Arizona's controversial law targeting illegal immigrants from going into effect, and said the federal government is likely to be able to prove the measure is unconstitutional.

Republican Governor Jan Brewer had asked the appeals court to lift an injunction imposed in July 2010, the day before the law was to take effect. The U.S Justice Department sued to block the law, saying it violates the Constitution because enforcing immigration law is a federal issue. The court said the government is likely to succeed in its arguments that Congress has given the federal government sole authority to enforce immigration laws, and that Arizona's law violates the Supremacy Clause of the Constitution. However, attorneys defending the law argued that the federal government hasn't effectively enforced immigration law and the state law will assist federal authorities.

Among the parts of the law that remain blocked are: requiring police to question people's immigration status while enforcing other laws if there's a reasonable suspicion they're in the country illegally; requiring all immigrants to get or carry immigration registration papers; making it a criminal offense for an illegal immigrant to seek work or hold a job; and allowing police to arrest suspected illegal immigrants without a warrant.