Thousands of part-time health workers could win better pensions under a series of test cases to be heard in the House of Lords next week.

Unison general secretary Rodney Bickerstaffe is urging the government to accept the justice of the cases and drop the opposition inherited from the previous Conservative government.

'It is a question of fairness,' he said. 'New Labour, quite properly, say they are different from the previous government. This is an area where they can move in a different direction and signal that the work of part- time workers, particularly women, is valuable and valued by society.'

At least 100,000 part-time workers in the private and public sectors have lodged appeals against their exclusion from occupational pension schemes. If they win, it could cost employers 'tens of millions of pounds' in back pension contributions.

The Treasury has so far fought the cases, brought after a series of European Court of Justice decisions found that the exclusion of part-time workers from pension schemes was discriminatory, since most part-timers are women. The Treasury faces a huge bill for back NHS pension scheme contributions if the government loses, although there would also be gains from part- time workers losing eligibility to means-tested benefits.

The test cases have been rejected by industrial tribunals, employment appeal tribunals and the Court of Appeal. But a new European Court judgement in December has given Unison hope of a favourable Lords decision.

The court ruled that part-time mental health workers should have the same pension rights as full-time staff, and accepted that their pensions service should be backdated to 1976, instead of two years, the limit under British equal pay laws.

Mr Bickerstaffe said: 'Pensions are deferred pay. We think part-timers have been robbed by the system. The message to employers and the government is that workers should not be treated as second class because they are part-time.'

Although the claims were first lodged three years ago, the Lords rulings still represent the beginning of the process. They will determine time limits for starting and backdating claims. Industrial tribunals would then consider whether there were grounds other than sex for excluding part-time workers. After that, individual cases would be heard.

It could mean an extra pension of pounds750 a year and a lump sum of pounds2,250 for a part-time woman local government worker earning pounds6,000 a year. The worker would also have to pay back pension contributions, but would gain more in extra pension.