Opponents of same-sex marriage reform risk alienating a generation in Northern Ireland, a leading campaigner will say later.

Ged Killen, a gay Catholic and Labour MP from Glasgow, is married to a man from Northern Ireland.

He will address the annual Amnesty International Pride lecture in Belfast later and will say it is anti-democratic for Westminster to refuse to legislate on the matter in the absence of devolved government at Stormont.

“Make no mistake. Same-sex marriage in Northern Ireland will happen.

“Those who stand opposed risk alienating a generation.

“The children of future married same-sex couples, their friends and their families will not forgive nor forget those who stood in the way of equality.”

Northern Ireland is the only part of the UK in which same-sex marriage has not been legalised due to opposition from its largest party, the DUP.

The Republic of Ireland voted to allow the ceremony in 2015.

Theresa May’s Democratic Unionist allies hold that Christian marriage is between a man and a woman and have used a parliamentary mechanism at Stormont to veto efforts to legislate.

When the now-suspended Assembly was operating its members voted in favour of change while public opinion polls show a majority of above 70% back social liberalisation.

Mr Killen will say there is also a clear majority at Westminster who would vote to extend the right of marriage in Northern Ireland in the absence of a devolved Assembly in Belfast.

“The opponents of same-sex marriage seek to impose a minority view on the LGBT community in Northern Ireland through a technicality.

“Under these circumstances, it is anti-democratic not to legislate for same-sex marriage and Westminster has a moral and a democratic obligation to do so.

“Instead of facing a vote, they now run away, using cheap parlour tricks and the machinations of parliamentary processes and the petition of concern to deny the will of the people of Northern Ireland.

“It can only be described as one thing, a stitch-up to prevent people from claiming their rights.”

He urged the Prime Minister to decide whether her deal with the DUP to prop up the minority Government, which preserved her “fragile” grip on power, was worth perpetuating ongoing discrimination against the LGBT+ community in Northern Ireland.

Mr Killen has previously acknowledged that his relationship with religion had not always been an easy one.

In 2012 he wrote to the then head of the Catholic Church in Scotland, Cardinal Keith O’Brien, challenging him on anti-LGBT comments he had allegedly been making in the media.