Section 273 (application of Act to Crown employment) does not apply to employment in respect of which there is in force a certificate issued by or on behalf of a Minister of the Crown certifying that employment of a description specified in the certificate, or the employment of a particular person so specified, is (or, at a time specified in the certificate, was) required to be excepted from that section for the purpose of safeguarding national security.

A document purporting to be such a certificate shall, unless the contrary is proved, be deemed to be such a certificate.