A federal appeals court in San Francisco has ruled that a ban on same-sex marriage in California violates the United States Constitution.
A statewide referendum on the issue, known as Proposition 8, imposed a bar on gay unions in November 2008 but that was overturned by a court in 2010.
Now a three-judge panel of the 9th US Circuit Court of Appeals has ruled 2-1 that the latter judgement was correct.
Supporters of the ban previously vowed to challenge the panel's decision if it went against them.
It's now likely the case will go to the US Supreme Court.
However, proponents of Proposition 8 have the option of appealing first to the 9th Circuit Court, an 11-person panel.
Gay marriage is legal in six states in the United States.