5Any two or more local planning authorities may, with the approval of the Secretary of State, concur in establishing a joint advisory committee for the purpose of advising those authorities as to the preparation of structure plans and local plans and generally as to the planning of development in their areas ; and any such committee shall be constituted in such manner as may be determined by the authorities by whom it was established :
Provided that a majority of the members of any such committee shall be members of one or other of those authorities.