Cardinal André Vingt-Trois, Archbishop of Paris and head of the bishops' conference, said after discussing the draft law with Justice Minister Christiane Taubira that he wanted to prevent or weaken the planned reform.
He urged Catholics to press their elected officials "to organise the debate the Government doesn't want to organise."
Pope Benedict XVI told bishops from western France during their ad limina visit last week they should not shy away from the debate out of fear of appearing old-fashioned.
"Defending life and the family in society is not retrograde at all, but prophetic," he said.