A third bid to legalise same-sex marriage in Northern Ireland is set to fail after a contentious Assembly voting mechanism was again invoked by unionists.

The Democratic Unionists have tabled a petition of concern ahead of tomorrow's debate on a Sinn Fein motion to change marriage laws in the region, meaning the proposal can only pass if a majority of both unionists and nationalists support it.

With the DUP holding the most seats on the unionist side of the house, the move to replicate legislation already introduced elsewhere in the UK is doomed to fall.

Supporters of gay marriage held protests in Belfast and Londonderry tonight to object to the tabling of a petition on such an issue. The voting mechanism was incorporated into Assembly structures during the peace process to protect minority views.

But the DUP's Peter Weir stood by the move, saying: "The DUP as a party support the traditional definition of marriage as one man and one woman, and we will use whatever parliamentary devices we have at our disposal to make sure that remains the case - so we make no apology in taking a strong stand to defend marriage."

He said the only "abuse of process" was the introduction of the same motion for the third time in little over a year.

The first gay marriages took place in England and Wales last month. Scotland passed a similar law in February and the first same-sex marriages are expected there in October.

In Northern Ireland, with a greater proportion of Catholic and Protestant churchgoers than other parts of the UK and arguably a more conservative social culture, any change to the law would prove highly controversial.

A referendum on the issue is likely to be held in the Republic of Ireland next year.

Sinn Fein Stormont MLA Caitriona Ruane said other jurisdictions were moving forward to ensure marriage equality for all.

"The North should not be left behind," she said.

"Giving all couples equal marriage rights under the law does not threaten anyone's beliefs, religious or otherwise. Churches are free to define marriage as they wish but the state has a duty to treat all citizens equally."

Earlier, Catholic bishops urged politicians to reject "marriage equality" for same-sex partnerships in Northern Ireland.

An open letter from the senior clerics said the proposal undermined the principle of equality by applying it "inappropriately".

It added: "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."

In an open letter signed by the head of the Irish church, Cardinal Sean Brady, and six bishops, the Catholic leaders claimed the family, based on the marriage of a woman and a man, was "the best and ideal place for children and deserves special recognition and promotion by the state".

"We believe that the state should urgently provide more and better services in support of marriage in which mothers and fathers can provide the optimum loving and stable environment for children to grow and flourish," it said.

The letter said same-sex relationships are already comprehensively provided for in legislation through their recognition as civil partnerships and associated equality legislation.

The Church of Ireland has also restated its position, saying: "The Church of Ireland affirms, according to our Lord's teaching, that marriage is in its purpose a union permanent and life-long, for better or worse, till death do them part, of one man with one woman, to the exclusion of all others on either side.

"The Church of Ireland recognises for itself and of itself no other understanding of marriage."

The Stormont Assembly motion tabled by Sinn Fein will call on the DUP Minister of Finance and Personnel, Simon Hamilton, to introduce legislation to guarantee that couples of any sex or gender identity receive equal benefit.

The motion also said religious institutions should have the freedom to decide whether to conduct same-sex marriages.

National Union of Students leader Rebecca Hall said MLAs should back the motion.

She said: "This is a brilliant opportunity to show that Northern Ireland is moving forward, and politicians must not miss the chance to support equal marriage. It is crucial that politicians are on the right side of history as regards equality."