LOS ANGELES (AFP) - A federal judge on Friday struck down Utah's ban on gay marriage, saying that the western state's law - approved by voters in a referendum in 2004 - violates the United States (US) constitution.

Judge Robert J Shelby of Utah's Federal District Court, said the state's law conflicts with the right of same-sex couples to equal protection under the 14th Amendment to the constitution.

Gay groups welcomed the ruling, which comes after neighbouring New Mexico on Thursday became the 17th US state to legalise same-sex marriage.

"Few questions are as politically charged in the current climate," said Mr Shelby. "This... is especially true where, as here, the state electorate has taken democratic action to participate in a popular referendum on this issue.

"It is only under exceptional circumstances that a court interferes with such action," he wrote in the 53-page ruling, based on a lawsuit filed by three gay and lesbian couples who wish to marry in Utah, but are unable to do so.

"But the legal issues presented in this lawsuit do not depend on whether Utah's laws were the result of its legislature or a referendum, or whether the laws passed by the widest or smallest of margins."

He continued: "The question presented here depends instead on the (US) Constitution itself."

Utah's "current laws deny its gay and lesbian citizens their fundamental right to marry and, in so doing, demean the dignity of these same-sex couples ... Accordingly, the court finds that these laws are unconstitutional," he wrote.

Marriage laws are governed by individual US states, nearly 30 of which have amended their constitutions to ban same-sex marriage.

Efforts to give same-sex couples the same rights as heterosexuals have gathered steam in recent years.

One of the most important victories came in June when the US Supreme Court struck down the Defense of Marriage Act, which defined marriage as a union between one man and one woman.

Mr Moudi Sbeity, one of the plaintiffs in the Utah case with his partner Mr Derek L Kitchen, said the ruling "feels unreal".

"I'm just very thrilled that Derek and I will be able to get married soon, if all goes well and the state doesn't appeal. We want a farmer's market wedding because it's where we spend a lot of time," he said, cited by the Salt Lake Tribune.