UUP's Pauline Armitage says she will not be voting for David Trimble in tomorrow's critical vote for First Minister.
This means that without support from non-unionist members, Mr Trimble cannot be re-elected and elections may have to be called.
Mrs Armitage said the lack of detail over IRA decommissioning, changes to the RUC and her belief that the union was being destroyed had prompted her, after a lot of hard thinking, to decide not to support her party leader as First Minister.
With her vote, and that of fellow dissident, Peter Weir, he stood a chance of re-election, assuming that the PUP's two members decided to support him, which they have yet to do.
Without her vote, even if he gets Peter Weir's, he will only tie with the number of anti-Agreement unionist votes of the DUP and assorted hardline unionists.
Meanwhile, the Women's Coalition are trying to suspend standing orders to re-designate themselves as unionists so their votes would take Mr Trimble over the all-important 50% mark.