Nothing in Part I of this Act shall extinguish or abridge any powers or authorities which, if this Act had not been passed, would have been exercisable by virtue of the prerogative of the Crown, or any powers or authorities conferred on the Crown by any statute, and, in particular, nothing in the said Part I shall extinguish or abridge any powers or authorities exercisable by the Crown, whether in time of peace or of war, for the purpose of the defence of the realm or of training, or maintaining the efficiency of, any of the armed forces of the Crown.

Where in any proceedings under this Act it is material to determine whether anything was properly done or omitted to be done in the exercise of the prerogative of the Crown, . . .. . . F1 a Secretary of State may, if satisfied that the act or omission was necessary for any such purpose as is mentioned in the last preceding subsection, issue a certificate to the effect that the act or omission was necessary for that purpose; and the certificate shall, in those proceedings, be conclusive as to the matter so certified.