TORONTO – Canada’s Supreme Court ruled Thursday that gay marriage was constitutional, a landmark opinion allowing the federal government to call on Parliament to legalize same-sex unions nationwide.

If approved by a majority of the House of Commons, as widely expected, Canada would become the third country to embrace marriage by homosexuals and lesbians. Belgium and the Netherlands are the other two.

However, the court added that religious officials cannot be forced to perform unions against their beliefs. It also declined to answer a contentious question about whether traditional marriage between men and women also was constitutional.

Prime Minister Paul Martin said after the court’s ruling that since judges in six Canadian provinces and one of its territories already allow gay marriages, it should be approved nationwide. He said his government would introduce a bill shortly after Christmas.

He noted that members of Parliament would be free to vote their conscience, but his Cabinet ministers would have to support his bill.

“For many Canadians and many parliamentarians, this is a difficult issue involving personal and religious convictions and it represents a very significant change to a long-standing institution,” Martin said.

The court’s decision brings to the final stages a long, bitter fight over whether gays and lesbians should be allowed to marry in Canada. Public opinion is evenly divided on the matter, and advocates for both sides are preparing for the final phase of the battle.

“This is a victory for Canadian values,” said Alex Munter of Canadians for Equal Marriage.

He said that while public opinion may be split on the subject, Canadians endorse the Charter of Rights and Freedoms, the constitution’s bill of rights.

“One area of overwhelming consensus is that the charter is a document that Canadians cherish,” Munter said.

In the United States, gay marriage is opposed by a majority of Americans, according to an Associated Press-Ipsos poll taken in November, shortly after constitutional amendments in 11 states to ban same-sex marriage were approved.

“I think it’s a tremendously historic day that will help not just families in Canada, but people across the border who are wrestling with this question, of how the denial of marriage harms gay people and their loved ones,” said Evan Wolfson, executive director of Freedom to Marry, a gay-rights coalition based in New York.

“Canada is setting a standard for inclusion and fairness, and offering the real proof that ending discrimination helps families and hurts no one.”

Wolfson said of Canadians: “We now have the benefit of your wind in our sails, and we are charting the same course.”

In California, the Campaign for Children and Families called the Supreme Court opinion “shabby.” California’s state legislature last week began reconsidering a bill legalizing marriage licenses for gay couples.

To pass in the House of Commons, the legislation needs the approval of about 44 of the 95 Liberal backbench members of Parliament to obtain a 155-vote majority in the 308-seat House.

One top Liberal predicted the legislation should pass easily after its introduction, likely early next year. It already has the support of the 38-member Liberal Cabinet and virtually all the 54 Bloc Quebecois and 19 New Democrat lawmakers.