How can we achieve pay parity as equal pay is denied for equal work

I am a Direct recruited diploma engineer working with a  in Supervisory grade. I got selected after facing all India level written exam, followed by Interview and medicals. After getting appointment I had to undergo 1 yr in house training as a Supervisory trainee and finally absorbed as Supervisor grade II after passing Supervisor trainee examination. Whereas the eligibility (as per company policy) for a departmental candidate is ITI plus 16yrs of service, departmental interview unit wise and promoted to Supervisor grade II. Both Promoted and Directly recruited supervisors are entrusted with Identical of Duties and responsibilities, in the same department of the same Unit but the pay scale of Directly recruited is 30-40% less than the Promoted Supervisors. Kindly advice as to what can be done to achieve pay parity as Equal pay is denied for Equal work.