The GMB union may have to pay out £1m after more than 100 female workers won a sex discrimination case.
Scores of women council workers have won complaints of sex discrimination against their own union which took up equal pay claims on their behalf.
A tribunal ruled that the Middlesbrough Borough Council workers had been inadequately represented by the union in a dispute over equal pay, reports BBC News Online.
The tribunal said the union wrongly advised them to accept a deal instead of taking legal action – which could have resulted in higher compensation.
The union said it would appeal against the decision.
Brian Strutton, national officer of the GMB, said: “We are very surprised at the tribunal’s decision. The GMB has always been at the forefront of equal pay issues and we are adamant we have not discriminated against our members in Middlesbrough.
The women sought advice from the GMB after discovering that male employees doing work of equal value were getting paid up to 40% more because of bonuses.
The women won the original dispute with the council, which agreed to change its practices, last year.
The case is the latest twist in the growing number of equal pay claims affecting the public sector.