A ban on recognising gay marriage in law in Northern Ireland is to be challenged in Belfast high court later this week.

A gay couple who were married in England but now live in the region are taking legal action on Thursday morning aimed at getting equal status for their marriage.

The couple, who have asked to remain anonymous, will press the court to make a declaration that their marriage remains lawfully constituted in Northern Ireland and should be recognised as such. Reporting restrictions have been imposed in the court so as not to identify the couple.

The pair are backed by the Rainbow Project, one of the main gay rights organisations in Northern Ireland.

The region is the only part of the UK where there is a ban on recognising gay marriage in law. It is also the only part of the UK were same-sex marriages cannot be conducted.

John O’Doherty director of the Rainbow Project, said: “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.

“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.”

In April 2013 Unionist politicians defeated by 53 votes to 42 in the regional assembly a Sinn Féin attempt to create marriage equality for gay couples in Northern Ireland. The votes of the Democratic Unionist party and Ulster Unionist party in the Stormont parliament helped defeat the motion which was backed by the SDLP, Alliance and the Green party.

Gay equality issues have come back into public focus in Northern Ireland with the Democratic Unionist party trying to introduce a “freedom of conscience” bill into the assembly. Its critics say the law would allow evangelical Christians and other religious groups to discriminate against gay people.

The DUP put forward the bill at the end of last year in response to the Ashers Bakery controversy. The family run baking business refused to make a gay-themed cake for a same-sex couple and were reported to the Equality Commission of Northern Ireland.

• This article was amended on 14 January 2015 to correct the voting figures for the 2013 vote on marriage equality.