6(5)No employer or employers’ organization, and no person acting on behalf of an employer or employers’ organization, shall, so long as a trade union or council of trade unions continues to be entitled to represent the employees in a bargaining unit, bargain with or enter into a collective agreement with any person or another trade union or council of trade unions on behalf of, or designed or intended to be binding upon, the employees in the bargaining unit or any of them, and, if entered into, any such agreement is void.