Coventry City Council could be forced to pay up to £10 million in compensation to hundreds of women workers after a tribunal ruled it was not paying them fairly compared to male staff.
A total of 250 female staff including school cooks and supervisors and care home employees are in line to be awarded up to £40,000 each in what is believed to be a test case for future claims.
Union leaders for Unite warned the decision opened the door to a further 300 equal pay claims for women against Coventry - and others across the country.
Unite - created last year following a merger between Amicus and the Transport and General Workers Union - argued that the women workers were underpaid compared to bonuses received by male refuse collectors.The union said the women were receiving up to £140 a week less.
The tribunal, held in Birmingham, judged in its favour, meaning that women will be able to claim back pay for the last six years to a maximum of £40,000 each.
Coventry City Council is understood to be considering whether or not to appeal against the decision.
Tony Higham, the union's regional officer, said: "Our members have waited a long time for justice to be done as these claims were registered in 2005, but their resolve has never faltered," he added.
"We are now calling on Coventry Council to pay up. To appeal against this decision would not only be a further waste of rate-payers' money, but an insult to the women we depend on to feed our children and look after our elderly.
"Rate-payers may forgive them for wasting vast amounts of money defending these claims once, but I doubt they will again."
The tribunal heard evidence in three separate sessions towards the end of last year.
Unite claims it deliberately chose the most challenging cases to prove in the belief that if this one was won others would inevitably follow.
"We found the hardest case to fight first," said a spokeswoman for the union. "The fact that we won this means it is open for other claims at Coventry City Council and in other councils. There are other cases outstanding and we think they will be a lot easier to prove than this one, so this is a significant win."
More than 600 female care assistants and home carers at Warwickshire County Council learned they were to share millions under an equal pay settlement announced last December.
The county offered the money as recognition that the women had been unfairly paid less than men doing jobs of "equal worth".