Supporters of California's same-sex marriage ban have filed an appeal after a U.S. Federal Court judge overturned the controversial law.

The appeal was filed late Wednesday to the 9th U.S. Circuit Court of Appeals. Both sides had said they would appeal if they lost.

The appeal was filed by Protect Marriage, a coalition of religious and conservative groups that sponsored Proposition 8.

"This ruling, if allowed to stand, threatens not only Prop 8 in California but the laws in 45 other states that define marriage as one man and one woman," said Brian Brown, president of the National Organization for Marriage, which helped fund the 2008 campaign that led to the ban's passage.

The outcome in the appeals court could force the U.S. Supreme Court to confront the question of whether gays have a constitutional right to wed.

The appeal follows Chief U.S. District Judge Vaughn Walker's ruling Wednesday that the voter-approved ban was unconstitutional.

Proposition 8 outlawed gay marriages in California, five months after they were legalized by the state's Supreme Court. The proposition was approved by 52 per cent of California voters in a referendum and passed by the state legislature.

Two gay couples who said the ban violated their civil rights were plaintiffs in the challenge.

Proposition 8 "fails to advance any rational basis for singling out gay men and women," Walker wrote in his ruling. "[This law] does nothing more than enshrine … that opposite-sex couples are superior."

The ruling is significant in that it's the first such ruling in a federal court.

Despite the favourable ruling for same-sex couples, gay marriage will not immediately resume in California. The judge issued a temporary stay that stops his ruling from being implemented. He said he wants to decide whether to suspend his decision while it goes through the appeal process.

Massachusetts, Iowa, Connecticut, Vermont, New Hampshire and Washington, D.C., all grant same-sex couples the right to wed.

With files from The Associated Press