"The denial to the plaintiffs of the right to marry constitutes a denial of substantive due process," King County Superior Court Judge William L. Downing said in his ruling.

His decision is stayed until the state Supreme Court reviews the case, meaning no marriage licenses can be issued until then, said Jennifer Pizer, lead counsel in the case for Lambda Legal Defense in the case.

"Judge Downing saw the couples in the courtroom and he's recognized that they are full and equal citizens of Washington. No more and no less," Pizer said.

Washington is one of 38 states with laws defining marriage as a union between a man and a woman. Under a state high court ruling, Massachusetts has allowed gay marriage since May.

The Washington state couples challenged the state's Defense of Marriage Act, which restricts marriage to one man and one woman.

Arguing for the couples, attorney Bradley Bagshaw told Downing at a hearing last month that the act violates the state constitution by depriving same-sex couples of the same privileges and immunities as other residents, and by depriving them of life, liberty or property without due process of law.

Six couples filed the lawsuit in March after King County refused to grant them marriage licenses, and two other couples later joined the suit.

A second lawsuit was filed in April by the American Civil Liberties Union on behalf of 11 same-sex couples.