The Home Office has won its appeal against an immigration tribunal ruling that found people born in Northern Ireland are not automatically British, the woman at the centre of the case has said.
In 2015 Emma DeSouza, from Magherafelt, Co Londonderry, applied for a residence card for her US-born husband Jake. She made the application identifying herself as an Irish citizen.
The Home Office rejected the application on the grounds that it considered Ms DeSouza a British citizen.
Officials told her she should could either reapply identifying herself as British, or renounce her UK citizenship and reapply as an Irish citizen.
Ms DeSouza took a legal challenge against the Home Office and won, with a judge at a First Tier Immigration Tribunal ruling that she was an "Irish national only who has only ever been such".
The Home Office appealed against that decision at an Upper Tribunal hearing earlier this year.
Ahead of a scheduled press conference in Belfast, Ms DeSouza tweeted: "We have lost."
Ms DeSouza told a press conference: "We have not received a decision in our favour, the decision has gone in the favour of the Secretary of State.
"We have unfortunately lost. We are both deeply disappointed by this decision today."
The Co Londonderry woman insisted the Home Office position ran contrary to the 1998 Good Friday peace agreement, which gave anyone from Northern Ireland the right to identify as British, Irish or both.
Government lawyers argued the British Nationality Act 1981 was the relevant legislation - not the Good Friday accord.
It said the Act ruled that anyone born in Northern Ireland was automatically British, until such time as they renounce that citizenship.
Ms DeSouza has said her case will have implications for EU citizens post-Brexit.
She said the UK government had failed to implement the provisions of the Good Friday Agreement into UK domestic law.