TORONTO - The Supreme Court gave the go-ahead on Thursday to the federal government to introduce legislation that would redefine marriage nationwide to include same-sex couples, but it stopped short of ruling that the traditional definition of marriage was unconstitutional.

The decision was largely symbolic because the high courts of six provinces and one territory, representing 85 percent of the population, have already ruled that the traditional definition of marriage as a union between a man and a woman is discriminatory and unconstitutional. But the decision will give political cover to the government as it pushes legislation that has been promised for more than a year but remains controversial in the governing Liberal Party.

A decision by the Ontario Court of Appeals in June 2003 made Canada the third country, after Belgium and the Netherlands to permit gays and lesbians to marry. More than 3,000 same-sex couples have married in Canada since then, including many U.S. citizens.

Stephen J. Harper, leader of the Conservative Party and an opponent of changing the definition of marriage, said he viewed the decision as less than sweeping. "The court had plenty of chance to say there is only one definition of marriage, and it did not say that," he said, promising a fight in the House of Commons.

But gay rights groups welcomed the decision as a victory.

"The Supreme Court today gave a green light to the government's proposed equal marriage legislation, reflecting Canadian values and Canadians' commitment to fairness," Alex Munter, co-chairman of Canadians for Equal Marriage, said Thursday.