Thousands of women are expected to launch an equal pay claim against Sheffield City Council today.
The GMB trade union said the council's job evaluation scheme meant those in female-dominated roles - including cleaners, care managers and teaching assistants - were being underpaid.
GMB senior organiser Sue Wood said: ‘What we have uncovered in Sheffield is truly scandalous.
‘The council is using a rotten job allocation scheme that actively discriminates against its female workforce and opens up thousands of potential equal pay claims.'
Sheffield deputy leader Fran Belbin said: ‘Any member of staff can request a review if they believe their grade is not correct.
‘We encourage members of staff to raise any concerns about equal pay with us, so we can review and address if necessary.'