Catholic Bishops here have released a statement calling on politicians in Northern Ireland to reject an Assembly motion on same-sex marriage.

The bishops, led by Primate of All Ireland Cardinal Sean Brady, are asking MLAs not to "undermine the principle of equality" by applying it to what they describe as "two fundamentally different types of relationship".

They say that same-sex couples are already provided for by legislation in the North and all people are equal in the eyes of God.

However the bishops say they want marriage between women and men to be strengthened as a "unique and highly valued institution" and say the Marriage Equality motion should be rejected.

The Private Members' motion is due to be debated tomorrow.

In an open letter, the seven bishops say "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".

The bishops claim that the the proposed motion "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".

They say that this implies that both the biological bond and "natural ties between a child and its mother and father have no intrinsic value for the child or for society".

Back in April of 2012, the Assembly voted against calls to legalise civil marriage for same-sex couples in Northern Ireland by 53 votes to 42.