Information which does not fall within section 23(1) is exempt information if exemption from section 1(1)(b) is required for the purpose of safeguarding national security.
The duty to confirm or deny does not arise if, or to the extent that, exemption from section 1(1)(a) is required for the purpose of safeguarding national security.
A certificate signed by a Minister of the Crown certifying that exemption from section 1(1)(b), or from section 1(1)(a) and (b), is, or at any time was, required for the purpose of safeguarding national security shall, subject to section 60, be conclusive evidence of that fact.
A certificate under subsection (3) may identify the information to which it applies by means of a general description and may be expressed to have prospective effect.