The case involved a series of lawsuits seeking to overturn a voter- approved law that defines marriage as a union between a man and a woman.

With the ruling, California could become the second state after Massachusetts where gay and lesbian residents can marry.

"What happens in California, either way, will have a huge impact around the nation. It will set the tone," said Geoffrey Kors, executive director of the gay rights group Equality California.

California already offers same-sex couples who register as domestic partners the same legal rights and responsibilities as married spouses, including the right to divorce and to sue for child support. It's therefore unclear what additional relief state lawmakers could offer short of marriage if the court renders the existing ban unconstitutional.

A coalition of religious and social conservative groups is attempting to put a measure on the November ballot that would enshrine California's current laws banning gay marriage in the state constitution.

The Secretary of State is expected to rule by the end of June whether the sponsors gathered enough signature to qualify the marriage amendment, similar to ones enacted in 26 other states.

The cases before the California court were brought by the city of San Francisco, two dozen gay and lesbian couples, Equality California and another gay rights group in March 2004 after the court halted San Francisco's monthlong same-sex wedding march that took place at Mayor Gavin Newsom's direction.

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