The council has given communities & local government secretary Ruth Kelly until tomorrow to suspend her consultation on unitary proposals. If not, it will seek a judicial review at the High Court.
Chief executive Robin Hooper said the council might also seek an injunction to prevent Ms Kelly from continuing her consultation.
Shrewsbury & Atcham would be absorbed into a unitary Shropshire CC under Ms Kelly's plans.
The council said Ms Kelly's decision to consult was 'erroneous in law'.