Former First Minister, Mr David Trimble, did act unlawfully when he barred Sinn FΘin Ministers from attending cross-border ministerial meetings.
But after losing in the Appeal Court, Ulster Unionist Party lawyers are considering taking Mr Trimble's case to the UK's highest court, the House of Lords.
Today, the Appeal Court in Belfast ruled it was wrong of Mr Trimble to prevent Mr Martin McGuinness and Ms Bairbre de Br·n taking part in North-South Ministerial Council meetings last November in his bid to pressurise the IRA into arms decommissioning.
The appeal was dismissed by Lord Chief Justice, Lord Carswell, and Lord Justices Nicholson and McCollum on one main ground.
They declared it was wrong of Mr Trimble to refuse to endorse nominations already approved by the Deputy First Minister, Mr SΘamus Mallon, because the section of the agreement involved in the process was aimed solely at ensuring the working of the North- South Ministerial Council.
Lord Justice Carswell noted that at the time of the ban on Mr McGuinness and Ms de Br·n, Mr Trimble was under strong political pressure from his party to secure movement on weapons or withdraw from the Executive.
Mr Trimble's policy was to obtain that movement by refusing to nominate the ministers.
The judges rejected an "attractive argument" put forward during the appeal by Mr Trimble's lawyer, Mr Declan Morgan, that the ban was lawful because decommissioning itself was an "essential and intrinsic" overall objective of the agreement. They accepted the argument might be valid if arms decommissioning was the sole purpose, rather than one of a number of objectives, including "fostering North-South links" through cross- border bodies.
Lord Justice Carswell said he and his colleagues agreed that the judge who had first declared the ban illegal and against whose decision the appeal was mounted "was correct in his conclusion that the First Minister's refusal to nominate the applicants Ms de Br·n and Mr McGuinness must be regarded as an incorrect exercise of the discretion conferred upon him."