If before or during the determination, whether by the Minister or otherwise, of an appeal under section 23 of the principal Act (appeals against planning decisions) in respect of an application for planning permission to develop land, the Minister forms the opinion that, having regard to the provisions of sections 17(1), 18(1) and 38 of that Act (planning permission and industrial development certificates), of section 1(3) of the [1965 c. 33.] Control of Office and Industrial Development Act 1965 (office development permits) and of the development order and to any directions given under that order, planning permission for that development—
(a)could not have been granted by the local planning authority; or
(b)could not have been granted by.them otherwise than subject to the conditions imposed by them,
he may decline to determine the appeal or to proceed with the determination or, as the case may be, may direct that the determination shall not be begun or proceeded with.