Amnesty International has warned Northern Ireland that it will inevitably face a human rights legal case over its ban on gay marriage.
In February, the UK passed a bill in support of same-sex marriage, but Unionist parties have voted at Stormont to ensure Northern Ireland remains exempt.
Amnesty and the Rainbow Project say the LGBT community will use the Human Rights Act and the European human rights legislation to force Northern Ireland to follow Britain's law.
While the Northern Ireland Office said the gay rights issue was one for Stormont to decide, Patrick Corrigan, Amnesty's programme director in Northern Ireland, said states can not discriminate against gay couples in different parts of its territory.
"That obligation is clear in international law. This means marriage should be available to same sex couples in Northern Ireland just as soon as it will be to couples in other parts of the UK. There could be a straightforward legal challenge on the basis of inferior treatment of same sex couples in Northern Ireland with regards to the right to marry and found a family," he told the Guardian.
Rainbow Project director, John O'Doherty said that excluding Northern Ireland from the law gay marriage law would mean that if a married same-sex couple from England relocated there due to work or family reasons, their marriage would in effect be null and void.
"If a gay couple move to Northern Ireland their marriage is downgraded to a civil partnership," said O'Doherty. "This place is already struggling from a lack of inward investment compared to other parts of these islands so this anomaly makes our local economy even less welcome."
Gavin Boyd, the project's education and equality officer, said: "As long as there exists a legal inequality between Northern Ireland and Britain, there will be legal challenges. That has been the route that has been most successful in the past."