WASHINGTON (AP) -- A measure to let voters decide whether to ban same-sex marriages in D.C. cannot go on the ballot because it would violate a city human rights law, the Board of Elections and Ethics ruled Tuesday.

The D.C. City Council is expected to approve gay marriage next month, but opponents wanted voters to weigh in.

The elections board said allowing residents to vote on a ban would conflict with the city's 1977 Human Rights Act, which prohibits discrimination.

Errol R. Arthur, chairman of the two-member board, suggested as much at an October hearing. He said in a press release Tuesday that the "laws of the District of Columbia preclude us from allowing this initiative to move forward."

Similar ballot measures derailed same-sex marriage in Maine, where lawmakers passed a statute allowing gay couples to wed but voters repealed it, and California, where voters repealed court-sanctioned gay marriage.

A group called Stand4MarriageDC wanted a ballot measure that said "only marriage between a man and woman" should be "valid or recognized" in the city. The group's head is Bishop Harry Jackson, a Maryland church pastor who has been vocal in opposing the same-sex marriage law.

Cleta Mitchell, an attorney for the group, said members would appeal the ruling in D.C. Superior Court, likely within the week. She said the ruling was not a surprise and the measure does not violate the city's human rights act. She said the board made a mistake.

If the D.C. Council has the right to "change the law in order for same-same sex couples to marry," she said, "the people have the same rights ... to make law on the same subject."

This isn't the first time the board has cited the act in striking down a proposal to put a same-sex marriage issue before voters. In June the board rejected an effort to hold a referendum on whether Washington should recognize same-sex marriages performed elsewhere. The D.C. Council had previously voted to recognize those marriages.

Rick Rosendall, vice president for political affairs for the Gay and Lesbian Activists Alliance, said he was pleased with the ruling, though he recognizes it will be appealed.

"I'm very happy today," he said. "We're very, very happy about it."

Also on Tuesday the City Council set the first vote on its same-sex marriage bill for Dec. 1. The bill has to be voted on twice before passing. Four states -- Massachusetts, Connecticut, Iowa and Vermont -- allow same-sex marriage, and New Hampshire will join them starting Jan. 1.