Unison has lodged the largest equal pay claim in Europe on behalf of nearly 1,000 teaching assistants in Lancashire.
The claim could be worth nearly £10m a year in pay increases for the teaching assistants.
The union has lodged the claim with an employment tribunal in Manchester, but has put the application on hold while Lancashire County Council attempts to negotiate satisfactory gradings for its school support staff.
The claim is on behalf of 990 teaching assistants of the 4,000 employed by the authority, including nursery nurses, special support assistants, and learning and classroom assistants.
Unison national secretary for local government, Heather Wakefield said: "School support staff are among the most undervalued in local government. These equal pay claims demonstrate the need for local authorities to carry out urgent pay audits and bring the pay structures in line with the Equal Pay Act.
"By taking this case, Unison is demonstrating its commitment to attacking the gender pay gap and we will pursue further cases on behalf of all groups of women staff," she added.
The Lancashire claim is based on expert advice that the school support staff are employed to do work of equal value to the work of male comparators employed by the council in its environment directorate.
School support staff are among the lowest paid in the local government sector - 80 per cent earn less than £8,000 per year.
In local government, women earn on average 34 per cent less than their male counterparts.