Unions representing millions of workers including teachers, civil servants, council and NHS staff, will launch an appeal on Monday after losing a legal challenge to the way public sector pensions are calculated.
They will appeal against a High Court decision last year to approve the switch from the Retail Price Index (RPI) to the usually-lower Consumer Price Index (CPI) for the uprating of pensions.
Trade unionists will stage a protest outside the court in London ahead of the case.
Unite general secretary Len McCluskey said: "The change in how pensions are calculated is an attack on all workers. It represents a 15% increase in pension contributions for public sector workers, coming on top of wage freezes and spiralling cost of living, but make no mistake, where the public sector leads, the private sector will follow.
"Private sector employers are already attempting to move to the lower inflation index citing the Government's example. It is a scandal that across the economy, workers who have done the right thing and saved for their retirements are now looking at a much reduced pension income."
Brian Strutton, national officer of the GMB, said: "GMB consider that the High Court was wrong in fact and in law regarding this change to long-standing employment contracts which cuts pensions in payment by 15% and more.
"That is the reason GMB, with the other unions, is mounting this robust legal challenge to that decision."
Paul Noon, general secretary of the civil service union Prospect, said: "This is just one of the pension injustices inflicted on public and private sector pensioners by this mean Government. It will mean a permanent fall in standards of living for millions of people, breaking the promises made to them during the years they were saving for their pension. Not only will it drive many people into penury, by taking money out of people's pockets it will dampen the recovery from recession."
Public and Commercial Services union leader Mark Serwotka said: "The switch from RPI to CPI is an example of how this government wants public servants, pensioners and people entitled to benefits to pay the heaviest price for the recession. For new entrants to the civil service it means an immediate cut in their pensions, ripping up an agreement we reached just a few years ago.
"As well as challenging this in court, we are continuing to mount the widest, most co-ordinated industrial action we can to force the government to think again and show how out of touch millionaire ministers are with the lives and concerns of the rest of us."
Christine Blower, general secretary of the National Union of Teachers, said: "The Government's action is unfair to pensioners and its efforts to hide behind obscure legislation have fooled no-one. We look to the Court of Appeal to protect pensioners against this theft of their pensions."
Chris Keates, general secretary of the NASUWT teachers' union, said: "The switch from RPI to the lower CPI inflation measure will have a substantial and detrimental effect on the pensions of teachers and thousands of other public sector workers.
"I hope that the Court of Appeal recognises that teachers who entered into an agreement to save for their pension should continue to expect that the agreement will be honoured."Uni