The Greek Orthodox Church has expressed opposition to plans by the Athens government to give greater rights to unmarried couples.
Archbishop Ieronimos was elected leader of the Church in February
The Church's governing synod said it considered all common-law marriages to be tantamount to "prostitution".
The government proposes to give common-law couples the same rights as those who have gone through legal or religious ceremonies.
It wants to harmonise Greek law to European standards.
The new law would allow unmarried couples to make their relationship legally binding, by signing a simple notarial contract.
In its statement, the Church said that the proposal constituted a "catastrophic bomb" under the foundations of Greek society.
The leader of the Church, Archbishop Ieronimos, who was elected in February, had last week refused to comment on the government's plans, saying they came under the jurisdiction of civil authorities.
But the synod has now taken a different view.
The draft legislation is also opposed by Greece's homosexual community on the basis of discrimination, as it only takes account of heterosexual couples.