Amnesty International and gay rights pressure group the Rainbow Project have warned that the government in Northern Ireland will face legal action over its refusal to adopt same-sex marriage, once it passes into law in England and Wales.

The groups say it is inevitable that citizens in Northern Ireland will claim their government is infringing their rights.

The same-sex marriage bill which passed in the House of Commons in February would only make the practice legal in England and Wales if it becomes law later this year.

Scotland has its own same-sex marriage bill, a final version of which is due to be presented to the Scottish Parliament this summer, but Northern Ireland has not progressed on the issue since the UK’s adoption of civil partnerships in 2005.

A proposal put forward in the Stormont Assembly by Sinn Fein and the Green Party to allow same-sex couples to marry was defeated by the Democratic Unionist Party in October 2012.

The director of Amnesty International in Northern Ireland, Patrick Corrigan, said international law made it clear that the right of gay couples to marry could not be upheld in certain parts of a country’s territory while being banned in others.

He said Northern Irish citizens would be able to make “a straightforward legal challenge on the basis of inferior treatment” to fight for marriage equality to become law.

Rainbow Project director John O’Doherty warned that married gay couples would be deterred from moving to Northern Ireland in future, as their marriage would become void, and added: “[Northern Ireland] 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”.

A Northern Irish lesbian couple, Grainne Close and Shannon Sickles, were the first in the UK to be joined in civil partnership.

They said Belfast had been “proud” to host the country’s first legal union of a gay couple, and that they felt it was a shame Northern Ireland had failed to keep up its progress in gay rights following their landmark ceremony.

“In order for us to exercise our right to marry, we would have to leave Northern Ireland,” they said. “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.”

In a case that could echo the future legal battle over same-sex marriage, in October 2012 a High Court judge took the side of the Northern Ireland Human Rights Commission and ruled that Northern Ireland’s ban on gay couples adopting was unlawful, bringing it in line with the rest of the UK.

The majority of Northern Irish MPs who took part in the 5 February Commons vote on same-sex marriage in England and Wales voted against it.