Sinn Fein Ministers Mr Martin McGuinness and Ms Bairbre de Brun were given the go-ahead yesterday to challenge the ban on them attending meetings of the North-South Ministerial Council.
In the High Court in Belfast the two Ministers were granted leave to seek a judicial review of the First Minister's ban until the IRA re-engages with the arms decommissioning body.
The Ministers contend Mr David Trimble, who announced the measure at a crucial meeting of the Ulster Unionist Council, is acting unlawfully as he does not have the power to withhold their nominations.
Mr Justice Kerr's decision created a problem for Mr McGuinness, who was present briefly at yesterday's hearing. A full hearing will not take place until next week at the earliest and the Education Minister is due to meet his Southern counterpart in Dublin tomorrow. He now has to decide if he will defy Mr Trimble and proceed with the meeting.
Yesterday's court application began with a move by Mr Declan Morgan QC, representing Mr Trimble, to adjourn the case, in which the Deputy First Minister, Mr Seamus Mallon, is also a respondent.
Mr Morgan said Mr Trimble and Mr Mallon were entitled to bring their political judgment to the process and any question of illegality arose only if they moved outside that.
Mr Justice Kerr refused an adjournment.
Mr Seamus Treacy QC, for Ms de Brun, said the First Minister was frustrating the Belfast Agreement as the North-South councils were an integral part of the legislation.
"So far as he has taken into account the IRA's failure to reengage with the decommissioning body, it was an entirely irrelevant legal consideration - even though it may be politically relevant," he said.
"In exercising his power in this manner, the First Minister has abused his duty by imposing a sanction he is not legally entitled to impose."
Mr Michael Lavery QC, for Mr McGuinness, said it was not open to the First Minister to use his powers to advance one section of political interest - the Ulster Unionist Party.
Mr Michael Keogh, for Mr Mallon, said the application should have been directed at the Secretary of State, who had power to act if he felt Ministers were in breach of their duty.
Mr Justice Kerr said such a power was permissible rather than mandatory and therefore did not provide the applicant Ministers with an effective remedy.
Granting leave, he emphasised it was not an indication of the outcome but merely showed there was an arguable case worthy of further investigation.
Counsel for Mr McGuinness and Ms de Brun pressed for an early hearing.
The judge said they had to be realistic when dealing with matters of "considerable constitutional importance".
He directed that replying affidavits be filed by next Monday, and he said he would sit again on Tuesday to fix a date for the hearing.