Test cases affecting the pensions of thousands of part-time health workers were referred to the European Court of Justice by the House of Lords last week.
The cases follow a European court decision in 1994 that excluding part- time workers from company pension schemes may be discriminatory because most part-timers are women.
The test cases seek a ruling that claims should be backdated to 1976 instead of the two years allowed under British equal pay laws. That has been rejected by industrial tribunal, employment appeal tribunal and court of appeal hearings.
But last December, the European court ruled that two part-time mental health workers in Northern Ireland could backdate their claims to 1976.
Unison general secretary Rodney Bickerstaffe said he was delighted at the Lords' decision and 'very confident that the European Court of Justice will see the merits of our case'.
The cases potentially affect an estimated 100,000 part-time workers in the private and public sectors. Employers, including the Treasury, would have to pay 'tens of millions of pounds' in back pension contributions if the part-timers win their case.
The Treasury is responsible for NHS pension scheme contributions, but its bill if the government loses would be offset by part-time workers losing some means-tested benefits eligibility.
The European Court is expected to take up to 18 months before hearing the claims, which were first lodged more than three years ago.