Unions have lodged an appeal against a High Court decision allowing the Government to switch the uprating index for pensions and social security benefits from RPI to CPI inflation.
Four trade unions and a pensioner group joined forces to challenge the move, arguing that CPI was lower than RPI, leading to big cuts in workers' pensions.
Paul Noon, general secretary of the Civil Service union Prospect, said: "The CPI switch is just one of the measures the Government is seeking to introduce as part of its pensions reforms. It must be challenged as it would cut the income of our members in retirement, in both the public and private sectors, by up to a quarter.
"We remain committed to reaching an agreement that gives fair and sustainable pensions to all our members."
Jonathan Baume, leader of the FDA, which represents senior civil servants, said: "We have always said that we will use every avenue available to challenge this decision. The court gave us leave to appeal on two areas and we will be appealing both. We will continue to challenge where it makes sense to do so and where there is a reasonable prospect of success.
"We have always argued that any decision to change the index should be included within the overall negotiations on future pension arrangements. The Government has refused to even consider this, which leaves us with no option but to continue our legal fight in the courts.
"The uprating mechanism change from RPI to CPI means a real cut to our members' pensions. It comes on top of increased pension contributions and a two or three-year pay freeze, followed by a 1% pay cap."
© 2012 Press Association