OTTAWA (Reuters) - Canadian religious groups took the unusual step on Monday of asking the Supreme Court to allow them to challenge a lower-court ruling which overthrew the definition of marriage as a union of a man and a woman.

It would be unusual for such groups to be allowed to take over a case and carry it on to the Supreme Court, but they argue that the federal government abandoned its responsibility when it decided against appealing the Ontario court ruling.

"We're here because they've abdicated their role," Michael Martens of the evangelical group Focus on the Family Canada told reporters in the court lobby before the oral hearing.

The Supreme Court case was another battle in a long-running war over gay unions, which broke onto the forefront on June 10 when the Ontario Court of Appeal ordered that same-sex marriage be allowed in the province with immediate effect.

The issue has divided both Canadian public opinion and the Canadian Parliament, and it promises to feature in the federal election expected to be called next spring.

Both the federal government, now in favor of gay marriage, as well as the seven homosexual couples who had sought permission to marry opposed the right of the religious coalitions to launch an appeal of their case.

"The litigation is over. Five of our seven applicant couples are now legally married. We cannot turn the clock back," the couples' lawyer, Martha McCarthy, said in a statement.

Following the Ontario court decision in June, the appeal court in British Columbia extended gay marriage rights to that province and the federal government drafted its own legislation which would extend them to the entire country.

The government has also made a separate "reference" to the Supreme Court, asking its opinion on its draft bill, including whether it would be constitutional to allow gay marriage.

But the religious groups want a clear answer on whether the Supreme Court think the constitution actually requires, rather than allows for, gay marriage.

"The reference questions are extremely narrow and do not give the court an opportunity to really consider the constitutionality of the (traditional) definition of marriage itself," Janet Epp-Buckingham said for the Evangelical Fellowship of Canada.

The federal government's "reference" case before the Supreme Court is due to be heard next spring, coincidentally around the time of the expected federal election.