About 5,000 mainly female council staff have won their case for equal pay at an employment tribunal.
The workers include cleaners, cooks, care assistants and caretakers and were employed by Birmingham City Council.
Unions said the women may be owed £30m in back-pay for bonuses paid only to certain male workers. Solicitors said the pay-out could be up to £600m.
The Unison union said it was a "major" victory. The council said the ruling related to its old bonus structure.
Councillor Alan Rudge, cabinet member for equalities and human resources, said the council had removed its "inappropriate bonus schemes" in 2007.
He said it had introduced a revised pay and grading structure which was in line with the Equal Opportunity Commission equality guidelines.
Many people were unhappy with the new deal, which unions said left some people £18,000 worse off.
About 3,000 council staff went on strike in 2008 over the new pay structure, which Unison said still left women underpaid.
Theresa Daly, of Bordesley Green, Birmingham, has worked as a laundress at Saltley School for 25 years.
She said: "I was annoyed. I worked as hard as the men and I had been working a long time.
"To think they were getting paid more than me wasn't very fair.
"I am glad everything has been sorted and now that we are going to get something out of it, I am very happy."
Unison said it was difficult to calculate the total amount the council would have to pay out as a result of the tribunal.
Payments would be calculated on an individual basis depending on factors such as length of service and whether people were full-time or part-time workers.
Both Unison and the GMB union have estimated the pay-out could be in the region of £30m in total.
Stefan Cross of Stefan Cross Solicitors said it was a "massive victory" for all the workers.
He said the tribunal case had originally been brought on the grounds of gender pay discrimination.
Having established that a discriminatory pay system had been used by the council, he said everyone was now entitled to bring a claim, regardless of their gender.
He said 90% of the people affected were women, but his firm had also won claims on behalf of up to 100 male clients, who were employed in roles that had traditionally been considered women's roles such as caring and cleaning.
The tribunal ruled that bonus payments to men were discriminatory and typically allowed male employees to earn more than £50,000 a year.
The council had believed that refuse collection staff, who were on the same pay grades as women, deserved special treatment, Paul Doran of Stefan Cross Solicitors added.
He said the tribunal had found the payments could not be justified since they were being paid to the men for simply doing their jobs.
He also said the tribunal found senior council managers had been aware of the problem as early on as 2000 but had not done anything about it.
"The fact that Birmingham City Council simply failed to acknowledge it had a problem should act as a warning to other local authorities who continue to deny their female employees their basic rights," Mr Doran added.
Unison general secretary Dave Prentis said: "For too long Birmingham City Council has failed to live up to its responsibilities to pay these women workers fairly."
He said the council had cost taxpayers a huge amount in legal fees fighting the claims, which would have been better spent providing local services.