The Catholic church of the Diocese of Tonga and Niue says it does not support the Convention on the Elimination of any form of Discrimination Against Women, CEDAW.
This comes after the Tongan cabinet announced it would ratify the convention with certain conditions so as not to clash with local laws.
Radio Tonga reports that Cardinal Mafi wrote a letter stating the reasons for the church not supporting the move.
He says CEDAW was discussed in 2009 and was not passed by the Parliament.
The Cardinal says that CEDAW states to eliminate all forms of discrimination against women, which is already clear enough in Tonga's laws and is forbidden in Christian churches.
He says the nation's motto "God and Tonga are my inheritance" highlight fact that God is first.
The Cardinal says therefore everything has to go accordingly with God's plans and not what the United Nations wants.
He says the church does not approve of the convention because some of the clauses in it are in conflict with the church's beliefs, like abortion and same sex marriage.