Northern Ireland's assembly voted in favor of same-sex marriage but it was vetoed by the ruling party. The country remains the only part of the United Kingdom where civil marriage is not legal.
Northern Ireland's power-sharing assembly voted narrowly in favor of same-sex marriage for the first time on Monday.
The bill was set to be adopted by just one vote - with assembly members backing the motion by 53-52 - but the ruling Democratic Unionist Party (DUP) sank the proposal with a constitutional veto.
Under the so-called 'petition on concern' a vote can only pass if supported by 60 percent of voting members, including at least 40 percent of each of the nationalist and unions members voting.
The safeguard was introduced to preserve minority rights when Northern Ireland won devolved powers from London. But some analysts say the rule is being used too frequently and for the wrong reasons.
The bill's failure follows fractious debates over gay marriage equality with several commentators blaming Northern Ireland's evangelical christian community, which heavily influences DUP policies.
Campaigners say that same-sex couples are being discriminated against, while some christian groups claim that marriage cannot be redefined.
The proposed legislation, which follows the signing into law of similar legislation in the Republic of Ireland last week, was introduced by nationalist parties Sinn Fein and the Social Democratic and Labour Party.
LGBT groups undeterred
Monday's vote means the legislation has failed to win approval on five occasions and Northern Ireland remains the only part of the UK that has not passed a law to allow same-sex marriage. England, Scotland and Wales legalized the unions last year.
But gay rights activists have vowed to continue the fight to achieve equal rights.
‘It is true that the DUP has abused the petition of concern to block this vote and are now ignoring the will of the Assembly and the people of Northern Ireland but we will not allow them to dampen our joy today. Our campaign continues and it will not end until marriage equality is a reality for everyone in Northern Ireland," John O'Doherty, Director of The Rainbow Project said.
Amnesty International hailed the majority vote but said the Stormont government had "failed to keep pace with equality legislation elsewhere in the UK and Ireland."
An Ipsos MORI survey in July found that 68 percent of adults in the devolved territory were in favor of same-sex marriage, rising to 82 percent among 16- to 34-year-olds.
Later this month, courts in Belfast are to hear challenges to the marriage ban from at least three same-sex couples.
The previous four attempts to legalize same-sex marriage were rejected by slim margins.