Unison will seek permission to appeal to the Supreme Court after losing its challenge to the legality of employment tribunal fees.
The trade union is calling for a judicial review into the validity of the fees introduced by the coalition government for workers to take employment disputes to a tribunal.
The Court of Appeal dismissed Unison's appeal after an initial rejection in the High Court.
Figures show claims have dropped by 70% since the introduction of fees, which Unison claims is evidence enough that they are discriminatory and restricting access to justice.
However Lord Justice Underhill dismissed these arguments, saying that the fee system was not generally so unaffordable that there was no effective remedy for claimants under European Union law.
He also noted the government's commitment to a review of the fees regime, and observed: "The decline in the number of claims in the Tribunals following the introduction of the Fees Order is sufficiently startling to merit a very full and careful analysis of its causes.
But he did agree there may be some merit to the case. "If there are good grounds for concluding that part of it is accounted for by claimants being realistically unable to afford to bring proceedings, the level of fees and/or the remission criteria will need to be revisited," he added.
Unison general secretary Dave Prentis said the decision was a "huge disappointment", calling it a "major setback" for working people.
"There is stark evidence workers are being priced out of justice and it is women, the disabled and the low-paid who are being disproportionately punished.
"Our fight for fairness at work and access to justice for all will continue until these unfair and punitive fees are scrapped," he added.
Employment tribunal fees were introduced in July 2013 and start at around £160 but can escalate substantially.
Eversheds Partner Naeema Choudry said: "Although there is no prospect of fees being abolished following the review, we could still see a meaningful reduction in fee levels, especially given that even some employer groups have acknowledged that the current level of fees is too high."
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