A federal appeals court Tuesday struck down California's ban on same-sex marriage, clearing the way for the U.S. Supreme Court to rule on gay marriage as early as next year.

The 2-1 decision by a panel of the U.S. 9th Circuit Court of Appeals found that Proposition 8, the 2008 ballot measure that limited marriage to one man and one woman, violated the U.S. Constitution. The architects of Prop. 8 have vowed to appeal.

The ruling was narrow and likely to be limited to California.

PHOTOS: Prop. 8 ruled unconstitutional

“Proposition 8 served no purpose, and had no effect, other than to lessen the status and human dignity of gays and lesbians in California,” the court said.

The ruling upheld a decision by retired Chief U.S. District Judge Vaughn R. Walker, who struck down the ballot measure in 2010 after holding an unprecedented trial on the nature of sexual orientation and the history of marriage.

In a separate decision, the appeals court refused to invalidate Walker’s ruling on the grounds that he should have disclosed he was in a long term same-sex relationship. Walker, a Republican appointee who is openly gay, said after his ruling that he had been in a relationship with another man for 10 years. He has never said whether he and partner wished to marry.

FULL COVERAGE: Prop. 8

ProtectMarriage, the backers of Proposition 8, can appeal Tuesday's decision to a larger panel of the 9th Circuit or go directly to the U.S. Supreme Court. The high court is expected to be divided on the issue, and many legal scholars believe Justice Anthony Kennedy will be the deciding vote.

Gays and lesbians were entitled to marry in California for six months after the California Supreme Court struck down a state ban in May 2008. The state high court later upheld Proposition 8 as a valid amendment of the California Constitution.

While the Proposition 8 case was still pending in state court, two same-sex couples sued in federal court to challenge the ban on federal constitutional grounds.

RELATED:

Supreme Court may not hear Prop. 8 appeal

Same-sex marriage won't resume immediately

Ruling to have limited effect outside California

-- Maura Dolan in San Francisco

Photo: Opponents of Prop. 8 demonstrate outside of the U.S. 9th Circuit Court of Appeals on Tuesday in San Francisco. Credit: Justin Sullivan / Getty Images