The order dated 19.4.1986 revoking the permission was challenged before the High Court on four grounds, viz., (a) that the lessees had had no reasonable opportunity of showing cause against the action proposed in the notice dated 9.1.1986 and that an opportunity of an oral hearing had 193 been denied; (b) that the Vice Chairman, under the provi sions of the Act had no authority or power to revoke a permission once granted; (c) that, at all events, the les sees having incurred enormous expenditure on the development work, and having, on the strength of the permission granted earlier on 31.1.1985, altered their position substantially to their disadvantage, the Vice Chairman was estopped from revoking the permission on principles of promissory estop pel; and (d) that the grounds on which cancellation rested were themselves irrelevant and insufficient in law to sup port the cancellation.