No principal employer of an establishment to which this Act applies shall employ inter-State migrant workmen in the establishment unless a certificate of registration in respect of such establishment issued under this Act is in force:
     Provided that nothing in this section shall apply to any establishment in respect of which an application for registration made within the period fixed, whether originally or on extension under sub-section (1) of section 4 is pending before a registering officer and for the purposes of this proviso, an application to which the provisions of sub-section (3) of section 4 apply shall be deemed to be pending before the registering officer concerned till the certificate of registration is issued in accordance with the provisions of that sub-section.