An industrial tribunal in Birmingham decided it would not be possible for them to claim any pension beyond the two-year limit already established. The total backdated award could amount to pounds 95m, according to the TUC.
The claims - submitted on behalf of part-timers in the NHS, banking, education, the local authority sector and the power supply industry - are now likely to go before the Employment Appeals Tribunal.
The judgment by the Birmingham tribunal also indicated that some of the cases might be "out of time".
While the tribunal argued that applications should have been submitted within six months of the end of the contract of employment in question, James Goudie, counsel for the TUC, said the relevant period was the six months after a key European Court judgment in September last year that employers who barred part-time workers from pension schemes could be guilty of indirect sex discrimination. Proceedings had been started within that period, he said.
John Monks, TUC general secretary, said: "The TUC is determined that employers should not be let off the hook for past discrimination."