A human rights campaign group has intervened in a court battle over whether humanist marriages should be legally recognised in the North.
Liberty has lodged a submission to the Court of Appeal in Belfast pending the resumption of a high profile hearing involving model Laura Lacole and Leeds and Republic of Ireland footballer Eunan O'Kane.
The couple originally won a landmark case to have their humanist wedding in June recognised in law.
However, Northern Ireland's Attorney General John Larkin and Stormont's Department of Finance challenged the outcome of the High Court decision before the Court of Appeal.
Days before the ceremony, the appeal court judges adjourned the case but granted the couple interim permission to have a legally binding humanist ceremony.
The appeal case will resume next month, when the wider issue of the status of future humanist weddings will be argued.
Humanist marriages are already legally recognised in Scotland, but not in England and Wales. They are also recognised in the Republic of Ireland.
In legal arguments submitted on Friday, Liberty urged the Court of Appeal to uphold the High Court's ruling.
Martha Spurrier, director of Liberty, said: "This is a case of discrimination, plain and simple.
"All this couple is asking for is the chance for humanists to express their beliefs on one of the most important days of their life.
"The High Court's ruling was a landmark in rights protection - to take a step backwards would be a sad day for progress and fairness."
Ms Lacole and Mr O'Kane first launched the legal bid after learning their planned humanist wedding in Ballymena's luxury Galgorm Resort would not be recognised in law.
For such recognition, they were told, they would need to have a separate civil ceremony.
The couple took the case against the General Register Office for Northern Ireland and Stormont's Department of Finance.
Mr Larkin also participated in the hearing because it touched on devolved Stormont legislation.
In the original High Court judgment, Mr Justice Colton quashed the GRO's refusal to grant legal recognition, finding such a position breached the couple's rights under the European Convention.
Humanism is a non-religious belief system that rejects the concepts of a higher deity or afterlife.
Humanists adhere to a scientific view of the world and believe humans steer their own destiny.