SAN FRANCISCO (AP) - A federal appeals court has declared California's same-sex marriage ban unconstitutional, paving the way for a likely U.S. Supreme Court showdown on the voter-approved law.

A three-judge panel of the 9th U.S. Circuit Court of Appeals in San Francisco ruled 2-1 Tuesday that a lower court judge interpreted the U.S. Constitution correctly in 2010 when he declared the ban, known as Proposition 8, to be a violation of the civil rights of gays and lesbians.

The measure, which passed with 52 percent of the vote in 2008, outlawed same-sex unions just five months after they became legal in the state.

Lawyers for Proposition 8 sponsors and for two couples who sued to overturn the ban have said they would appeal to the Supreme Court if they did not receive a favorable ruling from the 9th Circuit.