Trade Unions say Welsh councils may face legal action unless their female employees are paid the same as men.
The GMB and Unison claim women working for councils have been under-paid compared to men in equivalent jobs.
In 1997, councils agreed to review wages and implement new pay structures, with the deadline falling at the end of this month.
The Welsh Local Government Association said councils were committed to equal pay but warned it would cost £300m.
The unions have warned that many councils in Wales could face legal action if they fail to introduce equal pay for women and men and have said they will encourage their members to begin legal action in areas where unequal pay persists.
John Phillips, of the GMB Union, said that "progress has been appallingly slow in some areas" and said some authorities had shown a "lack of will" to implement the agreement reached a decade ago.
The Welsh Assembly Government has made extra money available for equal pay for the future, but has said councils need to foot the bill for back pay.
Steve Thomas, chief executive of the WLGA warned the eventual bill could have an impact on council projects.
He said: "Big capital programmes such as the building of schools or bridge repairs or road repairs that might have to go on the back burner for a while in order for us to meet the cost the liabilities involved."
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