Four unions and a pensioner organisation have lodged an appeal against the High Court's ruling in favour of the government change the uprating of public sector pensions.
Since April, the government has used CPI rather than RPI to uprate public sector pensions . In December, the High Court ruled the decision was lawful.
Prospect, the First Division Association (FDA), GMB and National Union of Teachers, as well as the Civil Service Pensioners' Alliance, have lodged the appeal challenging the ruling.
Fellow unions Unison and Unite have lodged a separate appeal but all parties are coordinating their campaigns.
The unions will argu that CPI is not an appropriate indicator of inflation as, they claimed, it does not measure increases in prices but assumes consumers will switch to cheaper goods.
They will also argue that in making its decision the government put the desire to cut the deficit before its duty to consider the general level of prices.
Paul Noon, general secretary of 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."
Public sector unions are also negotiating with the government a set of reforms to the structures of their pension schemes, including retirement dates, contribution rates and accrual rates.
Three years at the Treasury
Address key mistakes
Letter from the frontline
Breaching SEC rules
In Leeds since 1969