Six unions embroiled in a bitter row with the Government over pension increases will make their case heard at the High Court.
After launching a petition at the court, the unions will now explain why they think that the way the Government calculates pension increases is "unfair" to millions of workers.
They are incensed because the Government now uses the consumer prices index (CPI) rather than the traditionally higher retail prices index (RPI) to calculate increases for public sector pensions.
This is a relatively new method, only announced by Chancellor George Osborne when he unveiled his June 2010 Budget. However, the unions argue the change was implemented without consultation or negotiation.
As CPI is approximately 1.2% lower than RPI, the unions believe that the loss to existing public sector pensioners will be as much as 15%, with those currently paying into career-average schemes also losing out.
The unions are challenging the Government because they feel that the move was not permitted under social security legislation, and that it reneged on assurances given by successive governments that RPI would apply.
This could also have life-changing implications for pensioners who used to work within the public sector, with some potentially struggling to buy or rent a retirement home because their pensions might now be lower than expected.
Fire Brigades Union general secretary Matt Wrack said: "The Government actions are unfair and, we believe, unlawful. This is a vicious attack on existing and future pensioners that could cost them tens of thousands of pounds."