Glasgow council chiefs walked into an equal pay catastrophe which could cost taxpayers up to £500m, despite being told eight years ago its pay scheme was discriminatory, it has been revealed.

Despite knowing of this conclusion by the Equality and Human Rights Commission (EHRC) – which had been investigating the council's pay policy since 2009 – the city continued to fight its female employees, discriminate against them, and rack up costs as it defended the controversial Workplace Pay and Benefits Review (WPBR) which had been put in place to stave off equal pay claims.

The public sector union Unison has now written to current Glasgow councillors, briefing them on the findings of the EHRC investigation ahead of a meeting of the City Administration Committee on Thursday where councillors are expected to confirm the scrapping of the existing pay policy.

Both Glasgow City Council and the EHRC had refused to make the watchdog's conclusions public, despite freedom of information requests from campaigners. However council leader Susan Aitken is understood to have insisted the findings be released as part of her commitment to negotiate a settlement and end the council's legal battle with 6,000 of its female workers who have brought equal pay claims.

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Glasgow only conceded the claims were valid after a series of reversals in the courts in February this year.

But Scotland's statutory equalities body, after a detailed, independent investigation, had told the council in 2010 that the pay scheme was invalid and discriminatory.

Part of the investigation looked at the WPBR and whether the council's scheme breached the 1970 Equal Pay Act.

In reports to the council in January and February 2010, the EHRC said that both the design and the implementation of the pay structure appeared to be unfair to women workers.

The first concluded “There must therefore be considerable doubt as to whether the design of the job evaluation system used by Glasgow City Council to underpin its Workforce Pay and Benefits Review complies with equal pay legislative requirements generally but also specifically in relation to Pupil Support Assistants employed by the Council".

The second report, agreed and went further, stating: "It is difficult to escape the conclusion that ... payments, which are paid disproportionately to men, are offsetting the loss of bonuses"

Although the pay review had improved the lot of women workers, it still left them earning less than men in similar roles, and denied them access to supplementary payments which were largely on offer only to men. However Glasgow officials continued to insist the pay scheme treated people equally.

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It is not clear whether the elected council members at the time were informed of the outcome of the 2009 investigation or its potential financial implications.

Unison claims the council should have recognised the Commission's expertise and worked with it to fix the policy.

Peter Hunter, Unison regional manager for Scotland, said: "The implications of the EHRC papers are very clear. WPBR is invalid, discriminatory and unsuitable to be relied on. The fact that elected members are only now considering the major implications of an investigation from 2009 to 2011 suggest that governance arrangements have failed."

"Why was the litigation allowed to continue for so many years when the council knew that several independent experts had made such critical findings about the merit of the scheme?"

GMB Scotland Branch Secretary Rhea Wolfson said: "The level of knowledge someone at the council had, and did nothing with, is enough to take your breath away. Eight years later here we are, women are still working under this scheme while the council has known for nearly a decade it was discriminatory. It is unbelievable."

A spokesman for Glasgow City Council said: “In 2009 the EHRC had a concern about the terms and conditions of Pupil Support Assistants. This report set out their concerns. After a period of negotiation we reviewed the terms and conditions of PSAs and ensured that they were being paid fairly. The EHRC wrote to us in 2013 confirming they were content with the new arrangements and closing their investigation.

“Earlier this year, the council’s City Administration Committee took a decision not to continue with litigation and we expect to begin the process of implementing a new pay and grading structure in the coming weeks.”

Council leader Susan Aitken said: "Today, I am bringing forward another paper to the Council to abandon the WPBR, the pay scheme within which discrimination was embedded. I want a commitment from the Council to implement a fair and just pay and grading scheme which pays equally for equal work and which our employees have confidence in.

"The City Government will continue to work with employee representatives to ensure we do not repeat the mistakes of a decade ago and expose ourselves to fresh inequalities."