A ban on recognising gay marriage in law in Northern Ireland is to be challenged in Belfast high court later this week in a long awaited move that could bring marriage equality to the whole of the UK.

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.

The anonymous pair will press the court to make a declaration that their marriage remains lawfully constituted in Northern Ireland and should be recognised as such under human rights laws.

Buzzfeed News reports that the case was filed last Thursday and 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.

John O’Doherty, director of The Rainbow Project, a Northern Irish LGBT group involved in the legal challenge, told the news website:

“We are very happy to support this important legal challenge. While same-sex marriage legislation in Westminster had many positive aspects, we believe that its provision forbidding the recognition of lawful same-sex marriages in Northern Ireland is irrational, contrary to principles of British constitutional law and incompatible with the European convention on human rights.Northern Ireland's ban on same sex marriage to be challenged in court

“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. 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.

“Marriage is a fundamental human right, which is now recognised in the UK as including same-sex couples. We will work to ensure that this right is realised for everyone in the United Kingdom and we are confident that marriage equality will be achieved in Northern Ireland.”

The Democratic Unionist Party remains strongly opposed to gay rights, and has blocked all attempts to introduce marriage equality in NI. In April 2013 Unionist politicians defeated a motion for equal marriage by 95 votes to 45 in the Stormont assembly.

With Northern Ireland remaining the only part of the UK not to have introduced same sex marriage it appears that already there are serious issues to be dealt with.

Last week a couple who entered into a civil partnership in Northern Ireland were told they were unable to convert their partnership to a marriage. Under the Marriage (Same Sex Couples) Act in England and Wales, and the Marriage and Civil Partnership (Scotland) Act in Scotland, couples in civil partnerships must convert to a marriage in the country in which it was first registered. Much to the dismay of the couple who were previously informed the wedding would go ahead.

The Northern Ireland marriage equality legal challenge is still at an early stage but similar cases based on human rights legislation have previously led to the legalisation of same-sex adoption in Northern Ireland and there are hopes this case could finally be a way forward.