Catholic bishops in Northern Ireland have urged all members of the NI Assembly to vote against a motion on marriage equality at the assembly tomorrow.
In an open letter, the bishops and Cardinal Sean Brady said that same-sex relationships are already provided for in NI legislation through recognition as civil-partnerships and associated equality legislation.
They asked MLAs not to undermine the principle of equality by applying it inappropriately to two fundamentally different types of relationship.
Last month, same-sex marriage became legal in Wales and England, while Scotland passed a bill legalising same-sex marriage in February.
A referendum in Ireland has been promised on the issue for next year.
The letter from the bishops states: "We write to you today out of concern that the 'Marriage Equality' motion undermines a key foundation of that common good.
"We say this not only out of religious conviction, but also as a matter of human reason.
"Religious and non-religious people alike have long acknowledged and know from their experience that the family, based on the marriage of a woman and a man, is the best and ideal place for children.
"It is a fundamental building block of society which makes a unique and irreplaceable contribution to the common good.
"It is therefore deserving of special recognition and promotion by the State.
"The proposed 'Marriage Equality' motion before the Assembly effectively says to parents, children and society that the State should not, and will not, promote any normative or ideal family environment for raising children.
"It therefore implies that the biological bond and natural ties between a child and its mother and father have no intrinsic value for the child or for society."