Amnesty International has warned that a legal challenge is likely to the Northern Ireland assembly's decision to block a gay marriage equality bill. A Sinn Féin motion to bring in gay marriage to Northern Ireland was defeated by 51 votes to 43. It was the third time in the past 18 months that Stormont has rejected same-sex marriage. But the human rights organisation said those who had used their veto in the assembly were "like latter-day King Canutes, trying in vain to hold back the tide of equality". Patrick Corrigan, Amnesty's programme director in Northern Ireland, said: "States may not discriminate with regards to the right to marry and found a family on the basis of sexual orientation or gender identity.

"That obligation is clear in international law. This means that marriage should be available to same-sex couples in Northern Ireland, just as it is now in England and Wales and will shortly be in Scotland."

Despite the Democratic Unionist party using a so-called petition of concern to block a Sinn Féin motion that would have brought Northern Ireland into line with the rest of the UK in terms of gay marriage equality, Corrigan said there was bound to be legal action. "With politicians continuing to block equality, it is now inevitable that same-sex couples in Northern Ireland will take a legal challenge on the basis of inferior treatment with regards to the right to marry and found a family," he said. Under the rules of the Stormont assembly, legislation cannot pass if the representatives of one community refuse to support a new bill, which ensures no one section of the divided populace can impose laws on the other. Before the vote the Catholic hierarchy wrote a letter to every assembly member urging them to reject the bill.

In their letter, 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, back 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 in effect silenced the young people he had abused. Brady never disclosed he knew anything about the much-criticised deal until it was revealed in a Dublin newspaper two years ago. The motion put forward now in the Northern Ireland parliament 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.