Washington - The legislature of the US state of Vermont on Tuesday voted to make same-sex marriage legal, overriding the governor's veto of a bill that allowed gay couples to marry.

Vermont becomes the fourth state to allow same-sex marriage, and the first to do so through the legislative process and not court action. It joins the states of Iowa, Massachusetts and Connecticut, where the courts intervened to make gay marriage legal.

The Vermont Senate voted 23-5 and the House of Representatives voted 100-49 to override Governor Jim Douglas' veto. The legislation goes into effect September 1.

"The struggle for equal rights is never easy," said Vermont Senate President Pro Tem Peter Shumlin. "I have never felt more proud of Vermont as we become the first state in the country to enact marriage equality not as a result of a court order, but because it is the right thing to do."

On Friday, the Midwestern state of Iowa became the third US state to allow same-sex marriage after its Supreme Court ruled that a 1998 law violated the state's constitution.

The 1998 law limited marriage to a union between a man and a woman.

"We are firmly convinced the exclusion of gay and lesbian people from the institution of civil marriage does not substantially further any important governmental objective," the court said.

"The legislature has excluded a historically disfavoured class of persons from a supremely important civil institution without a constitutionally sufficient justification."

The long-simmering issue of same-sex marriage is both politically and morally charged in the US. In last November's elections, California, Arizona and Florida passed gay marriage bans. In Arkansas, voters passed a law depriving gays of the right to adopt children.

New York recognizes marriages by gay couples entered into in another jurisdiction.

"This historic vote in the Vermont legislature reminds us of the incredible progress being made toward equality," said Joe Solmonese, president of Human Rights Campaign, the country's largest gay rights organization.

"Less than five years ago, lesbian and gay couples began marrying in Massachusetts ... This is a law that will strengthen families and give meaning to the promise of equal rights for all."