A group of public sector unions will begin a High Court challenge today over the way public sector pensions are calculated.
Using the consumer price index (CPI) instead of the traditionally higher retail price index (RPI) was "unfair" on millions of workers, union leaders said, according to the Press Association.
The move by Chancellor George Osborne, which came into effect in April this year, will cost public sector pensioners around 15% if CPI remains 1.2% lower than RPI, they said.
Fire Brigades Union (FBU) general secretary Matt Wrack said the government's actions could be unlawful.
"This is a vicious attack on existing and future pensioners that could cost them tens of thousands of pounds," he said.
Public and Commercial Services (PCS) union general secretary Mark Serwotka said the unions are mounting the "widest, most coordinated industrial action we have seen in our lifetimes."
A Treasury spokesman said: "Public service pensions will continue to provide protection against inflation and will remain among the very best available, providing a guaranteed pension level for all employees.
"CPI is already used by the Bank of England to set its inflation target and unlike RPI is designed to take account of the fact that consumers tend to shop around, switching to cheaper alternatives when prices for similar goods change."
As well as the FBU and PCS, other unions involved include teachers' union NASUWT, Prison Officers' Association, Unison, Unite, Prospect, the FDA, GMB, Police Federation, National Association of Retired Police Officers and the Civil Service Pensioners' Alliance.
Putting the tech into protection
Square Mile’s series of informal interviews
Fallout from Haywood suspension
Launching later in 2019
£80bn funds under calculation