Amnesty International and gay pressure groups have warned that Northern Ireland's power-sharing government will soon face a human rights legal case over its refusal to allow gay couples to marry.

Unionist parties have voted at Stormont to ensure Northern Ireland is the only part of the UK where lesbian, gay, bisexual and transgender (LGBT) people are excluded from the same-sex marriage bill, which was passed in the Commons in February.

Amnesty and the Rainbow Project say it is inevitable that the LGBT community will use the Human Rights Act and European human rights legislation to force Northern Ireland to bring the law into line with Britain.

Any legal action is likely to take place in Belfast – ironically, the first UK city to host a gay civil partnership ceremony when Shannon Sickles and Grainne Close sealed their union at Belfast City Hall in 2005. However, the law excludes them from returning to the building to get married. They said they were disgusted at how some politicians had blocked full equality for LGBT people.

Sickles and Close said that at the time of their civil partnership ceremony "people felt proud that Northern Ireland was in the media for something positive, after years and years of negative media coverage all over the world," but they were disappointed at the lack of progress. "In order for us to exercise our right to marry, we would have to leave NI. We've made a life for ourselves here in Belfast, and we should have the same rights afforded to us as other citizens and be able to make a choice about marriage for ourselves."

As late as 1982 gay sex was still a criminal activity in Northern Ireland because the country was not included in the UK's gay law reforms of the 1960s. Gay rights campaigner Jeff Dudgeon took a case to the European court of human rights in 1981 which forced the UK government, in the face of opposition from unionist politicians such as the Rev Ian Paisley, to decriminalise homosexuality in Northern Ireland.

The Northern Ireland Office said the gay rights issue was one for Stormont to decide. Sinn Féin, the SDLP, the Alliance and Green parties support the right of LGBT couples to marry but the Ulster Unionists and Democratic Unionists remain opposed.

Patrick Corrigan, Amnesty's programme director in Northern Ireland, said states can not discriminate against same sex 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."

The Rainbow Project said excluding Northern Ireland from gay marriage law meant that if a married LGBT couple from England relocated there due to work or family reasons, their marriagewould in effect be null and void.

"If a gay couple move to Northern Ireland their marriage is downgraded to a civil partnership," said Rainbow director John 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 moves towards greater unionist political unity, possibly even a single unionist party, "would only exacerbate that problem particularly if it is going down the line of an old school unionism". Boyd said there was an opportunity for a court challenge: "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 sucessful in the past."