The Catholic church has backed unionist politicians' moves to block marriage equality in Northern Ireland.

A Sinn Féin motion to introduce legislation that would allow gay people to marry in the region is likely to be defeated at the Northern Ireland assembly on Tuesday.

Both the Democratic Unionist party and the Ulster Unionist party are to introduce a so-called "petition of concern", which would ensure there was no cross-community support in the chamber for the marriage equality bill.

Under the rules of the Stormont assembly, legislation cannot pass if the representatives of one community refuse to support a new bill, thus ensuring that no one section of the divided populace can impose laws on the other.

Before the vote, the Catholic hierarchy wrote to every assembly member to urge them to reject the bill.

The Catholic bishops in Northern Ireland said: "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. As Pope Francis stated recently, 'we must reaffirm the right of children to grow up in a family with a father and a mother capable of creating a suitable environment for the child's development and emotional maturity.'"

Among those who drafted the letter was Cardinal Sean Brady, the leader of Ireland's Catholics, who came under fierce criticism from victims of clerical sex abuse. As a priest in 1975, Cardinal Brady was the note taker in a deal between young victims of paedophile priest Fr Brendan Smyth and senior members of the Irish Catholic clergy, which effectively silenced the young people he had abused.

Sinn Féin's motion states that religious institutions should have the freedom to decide whether or not to conduct same-sex marriages.

This is the third attempt to persuade members of the regional assembly to back same sex marriage in Northern Ireland. A year ago, they rejected a similar motion by 53 votes to 42.

Meanwhile, a group of gay Anglicans have criticised the Church of Ireland for voting against gay marriage.

Dr Richard O'Leary, of Changing Attitude Ireland, said that not all Anglicans supported the church leadership's conservative stance on the issue.