A LEADING member of the Roman Catholic clergy in France has said government plans to legalise same-sex marriage there would profoundly affect the balance of French society.

Speaking in the pilgrimage town of Lourdes, Paris Cardinal Andre Vingt-Trois called it a reform for the few, not the many. He urged Catholics to show their opposition by writing and speaking to their elected officials and through other "democratic means of expression".

His call to action, announced at an annual plenary meeting of the country's Catholic bishops, came as President Francois Hollande's left-wing government prepares to present its draft bill on gay marriage in Cabinet this Wednesday.

French faith leaders – mostly Catholic, but also Jewish, Muslim, Protestant and Orthodox Christian – and conservative politicians have mobilised against the proposed law, especially its provision to allow gay couples to adopt children.

Vingt-Trois, who called the planned reform "a fraud", said: "The presidential and legislative elections [earlier this year] did not give them carte blanche, especially not for reforms that profoundly affect the equilibrium of our society."

Citing the slogan of the pro-reform campaign, he said: "It will not be 'marriage for all', it will be the marriage of a few imposed on all."

Opinion polls say voter support for same-sex matrimony has slipped several points to under 60%, and to under 50% for gay adoption, as the opposition has ratcheted up its campaign.

A BVA survey published by Le Parisien newspaper said this was the first fall in support after a decade of rising acceptance for the two reforms. The paper said: "Opinion trends on the subject are clearly on the retreat."

Vingt-Trois, pictured, did not openly call for street protests against the law, though his reference to "democratic means of expression" might be taken to signal tacit approval of them.

Lay Catholic groups organised protests in 75 cities around France last month and have planned more for the middle of this month.

The Church could organise a large demonstration, but is wary of adopting too prominent a role in an emotional and political debate.

Some conservative politicians have spoken in favour of a large street protest in Paris and some mayors – the main officials who celebrate civil marriages – have said they would not preside over ceremonies for gay couples.

Far-right National Front leader Marine Le Pen and Jean-François Cope, who is campaigning to lead the main opposition UMP party, both backed proposals last week for a large national protest march in defence of traditional marriage.

Addressing parliamentarians, the cardinal said they should be able to vote according to their consciences.

He said: "We appeal to their sense of the common good, which cannot be reduced to the sum of particular interests."

Vingt-Trois accused the French government of trying to rush through the marriage reform without a broad debate about its implications, especially for children who would grow up without a clearly identified mother and father.

Reacting to growing criticism, the government has scheduled longer parliamentary hearings on the bill than were first planned, but it still aims to pass it by mid-2013.

If the law is passed, France would become the 12th country in the world to allow same-sex marriage.