Making history on Monday a majority of Northern Ireland's Assembly members voted in favor of same-sex marriage for the first time.

But the motion was immediately blocked by the Democratic Unionist Party (DUP) who submitted a "petition of concern" requiring that the proposal achieve a cross-community majority to pass, effectively vetoing it.

Forty one nationalists backed the proposal, but it was approved by only four unionists, ensuring it fell.

It was the fifth time same-sex marriage was voted on by the assembly, with the four previous votes falling on a simple majority basis.

The petition of concern procedure was created in the 1990's to safeguard the rights of minorities in the Assembly and after the vote critics said it was particularly egregious to watch it used to do the opposite.

Northern Ireland is the last part of the UK where gay marriage is still not legal.

Fifty-three ministers supported the motion, while 52 voted against. One minister, the Ulster Unionist Party (UUP) leader Mike Nesbitt abstained, while the SDLP's Alban Maginness absented himself for the vote.

Petitions of concern necessitate separate majorities of both unionists and nationalists in order to carry.

Patrick Corrigan, Amnesty International’s Northern Ireland Program Director, told the Belfast Telegraph that the vote is nevertheless a "significant milestone on the journey to marriage equality."

"The abuse of the petition of concern, to hold back rather than uphold the rights of a minority group, means that Stormont has once again failed to keep pace with equality legislation elsewhere in the UK and Ireland," he added.

The situation as it currently stands creates serious legal anomalies for LGBT couples whereby their marriage is reclassified as a civil partnership in Northern Ireland when they arrive there from other parts of the UK or Ireland.

Two court challenges to the ban will be now heard in the courts in Belfast in November and December.