The Iowa Supreme Court issued a unanimous ruling Friday finding that the state's same-sex marriage ban violates the constitutional rights of gay and lesbian couples, making Iowa the third U.S. state where gay marriage is legal.

In its decision, the court upheld a 2007 district court judge's ruling that the law violates the state constitution. It strikes the language from Iowa code limiting marriage to only between a man and a woman.

"The court reaffirmed that a statute inconsistent with the Iowa constitution must be declared void even though it may be supported by strong and deep-seated traditional beliefs and popular opinion," said a summary of the ruling issued by the court.

'To tell the truth, I didn't think I'd see this day.'—Nancy Robinson, in long-term lesbian relationship

The ruling set off a celebration among the state's gay-marriage proponents.

"Iowa is about justice, and that's what happened here today," said Laura Fefchak, who was hosting a verdict party in the Des Moines suburb of Urbandale with her partner of 13 years, Nancy Robinson.

Robinson added: "To tell the truth, I didn't think I'd see this day."

Richard Socarides, an attorney and former senior adviser on gay rights to former president Bill Clinton, said the ruling carries extra significance coming from Iowa.

"It's a big win because, coming from Iowa, it represents the mainstreaming of gay marriage. And it shows that despite attempts to stop gay marriage through right-wing ballot initiatives, like in California, the courts will continue to support the case for equal rights for gays," he said.

Court rules dictate that the decision will take about 21 days to be considered final, and a request for a rehearing could be filed within that period. That means it will be at least several weeks before gay and lesbian couples can seek marriage licences.

But Polk County Attorney John Sarcone said the county attorney's office will not ask for a rehearing, meaning the court's decision should take effect after that three-week period.

"Our Supreme Court has decided it, and they make the decision as to what the law is and we follow Supreme Court decisions," Sarcone said. "This is not a personal thing. We have an obligation to the law to defend the recorder, and that's what we do."

The case had been working its way through Iowa's court system since 2005, when Lambda Legal, a New York-based gay rights organization, filed a lawsuit on behalf of six gay and lesbian Iowa couples who were denied marriage licences. Some of their children are also listed as plaintiffs.

The state Supreme Court's ruling upheld an August 2007 decision by Polk County District Court Judge Robert Hanson, who found that a state law allowing marriage only between a man and a woman violates the state's constitutional rights of equal protection.

Around the nation, only Massachusetts and Connecticut permit same-sex marriage. California, which briefly allowed gay marriage before a voter initiative in November repealed it, allows domestic partnerships.

New Jersey, New Hampshire and Vermont also offer civil unions, which provide many of the same rights that come with marriage. New York recognizes same-sex marriages performed elsewhere, and legislators there and in New Jersey are weighing whether to offer marriage.

A bill that would legalize same-sex marriage in Vermont has cleared that state's legislature but may be vetoed by the governor.