A U.S. appeals court will rule Tuesday on the constitutionality of California's Proposition 8 ban on same-sex marriage, the court said.

The ruling on California's same-sex marriage ban, passed by voters in 2008, could set national policy, if the Supreme Court takes the case. Both sides have indicated they will appeal if they lose the case in the 9th U.S. Circuit Court of Appeals.

About 40 U.S. states have outlawed same-sex marriage, but the issue has been decided on a state-by-state basis.

(Reporting By Dan Levine and Peter Henderson; Editing by Sandra Maler)