PARIS - France's law prohibiting gay marriage does not violate the constitution, the country's top constitutional watchdog ruled Friday, all but challenging parliament to debate overturning the ban.

The decision by the Constitutional Council puts the politically sensitive issue at the doorstep of French President Nicolas Sarkozy's governing conservatives, ratcheting up the pressure as they face presidential and legislative elections next year. Gay rights activists quickly condemned the ruling.

The issue has bared a contradiction about France: The country retains a conservative strain on family values, but its image is often linked to love and romance -- and polls suggest openness to gay marriage is growing.

Corinne Cestino and Sophie Hasslauer, who have lived together for 15 years, have four kids and seek the right to wed, challenged the constitutionality of the French civil code's stipulation that marriage must be between a man and a woman.

The couple have registered as civil partners, receiving tax benefits and other financial advantages, but gay rights activists point out the status does not allow for inheritance rights or joint custody of goods, among other things.

In its decision, the Council noted that lawmakers had agreed that the "difference in situations of same-sex couples and couples made up of a man and a woman can justify a difference in treatment concerning family rights."

The panel said two articles in France's civil code that define marriage as the union of a man and a woman are "in conformity with the constitution."

"It's not up to the Constitutional Council to substitute its assessment for that of lawmakers," wrote the body, noting that its job is to simply rule on whether a measure abides by freedoms guaranteed by the Constitution.

Cestino and Hasslauer's lawyer, responding to the ruling in an interview with AP Television News, insisted they weren't disappointed -- and claimed some success.

"The debate is still open as the Constitutional Council sends the hot potato back to the politicians," Emmanuel Ludot said. The court did say the French have the right to forbid same-sex marriage, he acknowledged, but it also made clear that "politicians must take their responsibilities, so we send them back the ball by telling them, 'do your job'."

Cestino, speaking on France-Info radio, acknowledged the ruling was "clearly a big disappointment."

"French society is ready (for gay marriage)," she said. "The block rests with politician."

Polls suggest she is right. A poll published for Canal Plus TV found 58 percent of respondents believe gays should be able to marry -- a double-digit percentage increase from a similar poll in 2006 -- while 35 per cent believe they should not.

Young adults, women and those on the political left were among those most in favor, with the elderly and far-right sympathizers the most opposed. The phone poll of 950 adults by TNS Sofres agency was conducted Wednesday.

Seven European countries have legalized gay marriage: the Netherlands, Belgium, Spain, Norway, Sweden, Portugal and Iceland. In Britain, same-sex couples can form civil partnerships that give them the same legal protection, adoption and inheritance rights as married couples enjoy -- but not the label of "marriage."

The Socialist Party, which has sponsored several ill-fated bills in parliament that could clear the way for gay marriage, sought to step up the pressure on Sarkozy's conservatives.

In a statement, the party's national secretaries criticized "the growing gap between the changes in society and our laws."

Some left-leaning officials have tried to take matters into their own hands. A Green Party mayor in the southwestern town of Begles officiated over a wedding of two gay men in 2004 -- but the highest French court later annulled the marriage.