Unison has said it expects to win the test cases, being fought with four other unions on behalf of 100,000 part-timers. They claim pension benefits should be backdated to 1976, rather than only two years as allowed under current law.
Most of the 10,000 Unison members are from local government. A win for unions would pave the way for a change to regulations allowing part-timers to buy back their pension service.
Unions pursued the case to the House of Lords after losing in tribunals and the Court of Appeal. But Unison legal adviser Richard Arthur said they were confident of winning after the European Court of Justice ruling in the Magorrian case last year that a two-year limit on pension rights for Northern Ireland mental health workers was discriminatory.
'We're talking here about a lot of very poor people . . . this is a way we can make sure these people have in their retirement dignity and decency.'
He said it sent a signal to all part-timers, particularly women, that their work was valued.
Unison pensions adviser Glyn Jenkins said the total bill for authorities would depend on how many workers took up the chance to buy into past scheme rights. Paying back-dated contributions could deter many of the lowest paid.
The cost was 'potentially many, many millions' but would be spread across more than 100 local authority funds, he said.
Principal pensions adviser to the UK Steering Committee on local government pensions, Bob Gibbs, said the case could affect up to 30,000 council workers.
But he added that there was genuine doubt over whether the Magorrian judgment set a precedent.
If part-timers did win retrospective rights, local government would wait to see what tack was taken by other statutory schemes, he said. The case starts on Monday and is expected to run for six days.