A same-sex couple in Northern Ireland have launched a long-awaited legal challenge to the region's continued ban on same-sex marriage in a move that could bring marriage equality to the whole of the UK.

The pair have filed a case in Belfast's High Court, where they will argue that the continued ban on same-sex marriage breaches human rights law. England, Wales, and Scotland began allowing same-sex marriages last year, leaving Northern Ireland as the only part of the UK where they remain illegal and unrecognised.

The couple, who are bringing the case anonymously, currently reside in Northern Ireland but were married in England last year. Despite being legally married in one part of the UK, in the eyes of Northern Irish law they are only considered to have a civil partnership.

John O'Doherty, director of The Rainbow Project, a Northern Irish LGBT group involved in the legal challenge, said this state of affairs is "irrational, contrary to principles of British constitutional law, and incompatible with the European Convention on Human Rights".

"We are resolute in our assertion that no-one can be married in one part of the United Kingdom and then not married in another," he added. "Once a couple is lawfully married in the UK, we contend that their relationship cannot be reclassified as a civil partnership without their consent ,which is exactly what the law currently does. The legislation says to lawfully married people that they are no longer married. This is unconscionable and cannot be permitted to continue."

Campaigners are understood to be building their case on the basis that the UK remains a single unitary state with administrative powers devolved to regions, rather than a federal republic such as the US. On this basis they will argue it is not possible for a couple to be considered to be married in one part of the UK but not in another.

The case was filed last Thursday and Buzzfeed News understands that campaigners are preparing for a legal fight lasting up to 18 months, with the potential for appeals that could take it all the way to the UK's Supreme Court, or even the European Court of Human Rights in Strasbourg.

Same-sex marriage is an issue devolved from Westminster to Northern Ireland's assembly, where politicians have stopped three attempts to legalise such ceremonies in recent years. Politicians in the unionist DUP, which was heavily involved in the "Save Ulster From Sodomy" campaign in the 1970s, have repeatedly blocked any attempts to legalise same-sex marriage in the assembly, invoking a procedural device called the "petition of concern" that effectively gives them a veto over any legislation.

Petitions of concern were designed as part of the Northern Ireland peace agreement to ensure no laws were passed exclusively by politicians representing either the nationalist or unionist communities. LGBT campaigners argue that using such a procedure on a matter that affects individuals in both communities is inappropriate.

A DUP spokesman confirmed the party's position has "not changed" and it continues to oppose same-sex marriage.

The Republic of Ireland will hold of a referendum on legalising same-sex marriage later this year. Current opinion polls suggest the public is likely to vote in favour of legalisation, potentially leaving Northern Ireland as the only major part of the British Isles where same-sex marriage is illegal and unrecognised.