The most serious legal and constitutional challenge to the continued implementation of the Belfast Agreement will be heard today by the House of Lords.
The action, taken by the Democratic Unionist Party, has the potential to present the British government with a political and legislative nightmare, and could yet pose the most serious crisis for unionism within the devolved administration at Stormont.
The DUP is seeking to have the Northern Ireland Secretary, Dr Reid, held in breach of his statutory obligation to call fresh Assembly elections last November following Mr David Trimble's failure to secure a majority of unionist votes in his first attempt to secure re-election as First Minister.
Mr Trimble was subsequently restored to the position he had earlier quit following the decision of a number of Alliance Assembly members to redesignate themselves as "unionists" for the purpose of the election.
However, the intervening weekend saw the expiry of the sixweek deadline set in legislation either to have a vacancy filled or for fresh elections to be called.
Counsel for the DUP will contend, not only that Dr Reid was in breach of his legal obligation, but that Mr Trimble and the SDLP leader, Mr Mark Durkan, were "invalidly" elected as First and Deputy First Minister and are, as Mr Peter Robinson, the DUP deputy leader, described them last night, "impostors" in their respective roles at the head of the power sharing Executive.
The British government will vigorously defend its position, and will be represented at today's hearing by the Attorney General.
While officially "confident" they will win the argument, it is understood ministers and officials have already framed emergency legislation to bring before the Commons should the action succeed.
Mr Robinson told The Irish Times last night that, in that event, the DUP would accept the need for retrospective legislation to validate administrative and other decisions made in the intervening period by the First and Deputy First Ministers.
However, he made it clear that that should not extend to any attempt to validate Mr Trimble's election at the second attempt courtesy of his deal with the Alliance Party.
That, he said, "would have very clear implications for democracy and for the accountability of the Assembly" and "would amount to the British government taking unto itself the right to appoint a First and Deputy First Minister, which is the right of the Assembly, whose mandate anyway was exhausted when we won the majority of unionist votes".
If the British government's actions are upheld by the Law Lords, Mr Robinson confirmed that the DUP would continue on its present course and seek a mandate in next year's elections to force a renegotiation of the Belfast Agreement.
He declined to comment on a possible DUP withdrawal from the Assembly, should they win their court fight but find themselves overruled by subsequent legislation.