(a) No employer having employes subject to any provisions of this section shall discriminate, within any establishment in which such employes are employed, between employes on the basis of sex by paying wages to employes in such establishment at a rate less than the rate at which he pays wages to employes of the opposite sex in such establishment for equal work on jobs, the performance of which, requires equal skill, effort, and responsibility, and which are performed under similar working conditions, except where such payment is made pursuant to (1) a seniority system; (2) a merit system; (3) a system which measures earnings by quantity or quality of production; or (4) a differential based on any other factor other than sex: Provided, That any employer who is paying a wage rate differential in violation of this subsection shall not in order to comply with the provisions of this subsection, reduce the wage rate of any employe.

(b) No labor organization, or its agents, representing employes of an employer having employes subject to any provisions of this section, shall cause or attempt to cause such an employer to discriminate against an employe in violation of subsection (a) of this section.