Campaigners are calling on South Lanarkshire Council boss Eddie McAvoy to quit after they say secret files show irregularities in payrolls and an attempted cover-up by the authority.
The council, which has more than 10,000 female staff compared to 4,400 male, is already facing a £10 million tribunal by female employees but now could face another multi- million-pound claim.
The SNP has demanded “full disclosure” from Mr McAvoy on the issue.
It has been claimed that the documents released by a court show that about 580 male gardeners and refuse collectors should have earned between £8.77 and £9.17, the same as 2,000 female care assistants.
But campaigners say that about 530 of the men were given extra bonuses and secret pay deals boosting their pay packets significantly.
They also claim that about 400 men were actually earning more than £10.07 an hour. The figures have emerged from a long-running freedom of information (FoI) case launched in 2010 by equal pay campaigner, Mark Irvine.
In light of many other councils paying male staff more than female peers, and being forced to pay compensation, Mr Irvine asked South Lanarkshire about male workers on the “LSO3” grade.
In theory, women should have been paid the same as the men on this job scale. However, in practice almost all men got more, the files indicated.
The council only released the documents after losing an appeal in the UK Supreme Court.
The authority refused to release the information when ordered to do so by the Scottish Information Commissioner and the Court of Session.
It is the first Scottish public body to ever appeal an FoI in the UK Supreme Court and will cost the council £200,000 legal fees.
But they potentially face paying out a lot more if evidence is produced of widespread unfair pay.
South Lanarkshire SNP councillor Peter Craig said: “There has got to be full disclosure. There are huge sums of money at stake.”
A South Lanarkshire Council spokesman said: “All council employees are paid in accordance with their evaluated rate, regardless of gender.
“Mark Irvine’s analysis is inaccurate – it ignores additional aspects of pay which are included in employee’s salaries, such as shift working and overtime.”
Earlier this year, more than 200 female nursery workers employed by Dumfries and Galloway Council demanded equal pay with their male counterparts.