A US federal appeals court struck down a California law on Tuesday that strips gays and lesbians of the right to marry in the latest round of America's long-running battle over same-sex marriage.

The Ninth Circuit Court of Appeals upheld a lower court judge's ruling that an amendment to the California state constitution banning same-sex marriage violated principles of due process and equal protection under the law.

Gay marriage was briefly authorized in California in 2008, but later banned by a referendum on what was known as Proposition 8. It rewrote the state's constitution to restrict marriage to unions between a man and a woman.

On Tuesday, the three-judge appeals panel in San Francisco ruled 2-1 that a lower court had properly declared the ban a violation of the US Constitution.

The ban had violated equality laws "to target a minority group and withdraw a right that it possessed, without a legitimate reason for doing so," Judge Stephen Reinhardt wrote in the majority opinion.

"Proposition 8 serves no purpose, and has no effect, other than to lessen the status and human dignity of gays and lesbians in California," the opinion said. "The constitution simply does not allow for laws of this sort."

In a dissenting opinion, Judge Randy Smith argued that the government could have a legitimate interest in preventing same-sex couples from marrying.

The ruling was relatively narrow in scope and did not address whether gays and lesbians have a broader constitutional right to marry.

The court's judgment drew sharp criticism from Republican presidential candidates.

"Today, unelected judges cast aside the will of the people of California who voted to protect traditional marriage," said Republican presidential frontrunner Mitt Romney.

"This decision does not end this fight and I expect it to go to the Supreme Court. That prospect underscores the vital importance of this election and the movement to preserve our values."

Former House speaker Newt Gingrich said the court's decision shows "more and more Americans are being exposed to the radical overreach of federal judges and their continued assault on the Judeo-Christian foundations of the United States."

Former senator Rick Santorum, another Republican White House hopeful, said the ruling ‘is another in a long line of radical activist rulings by this rogue circuit’ court.

However, supporters of gay rights welcomed the ruling.

California Attorney General Kamala Harris called the decision "a victory for fairness, a victory for equality and a victory for justice."

The decision did not give gay and lesbian couples the immediate right to wed while the ban remains in force until the US Supreme Court rules on a likely final appeal.

Joe Solmonese, president of Human Rights Campaign, said the decision overturns a ‘hateful law,’ and added: "This monumental decision affirms what we all know to be true: our constitution exists to protect the basic civil rights of all Americans - gay, straight, lesbian, bisexual, or transgender."

All three judges agreed the federal judge who previously found the voter-approved ban unconstitutional had no obligation to disclose that he was in a long-term gay relationship or recuse himself from the case.

Gay marriage is legal in six of 50 US states so far - Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and New York as well as in the federal capital city Washington - and is a hot-button political issue nationally.

At the White House, President Barack Obama's spokesman Jay Carney declined to comment on the specifics of the case, but noted that the president has "long opposed divisive and discriminatory efforts to deny... rights and benefits to same-sex couples."

David Cruz, a University of Southern California law professor, said the ruling may not be the final word on same-sex marriage rights.

He said the ruling was "very narrow and pretty much only applies to California's Proposition 8, and doesn't reach any other marriage restriction in the country."