UK CPI inflation fell from 4.8% in November 2011 to 4.2% last month, the equal largest drop since December 2008, according to the Office for National Statistics.
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.
The average income of households with children will drop by 4.2% - or £1,250 - a year between 2010/11 and 2015/16 as a result of the coalition's tax and benefit changes aimed at reducing the deficit, a charity has calculated.
UK CPI inflation fell from 5% to 4.8% in November, the Office for National Statistics has said, in line with economists' forecasts.
The government's decision to switch from RPI to CPI to calculate annual public sector pension upgrades has been ruled lawful in the High Court.
Consumer Prices Index inflation could fall to 3% by the end of Q1 2012 as the impact of the VAT hike and high oil prices falls away.
The government could to save almost £1bn if it altered the way in which it indexes state pensions.
The government will break its promise to "triple lock" the amount by which the state pension is adjusted for inflation, the National Pensioners Convention (NPC) has said.
Unions in the court battle over public sector pension reform will claim the government's action breaches workers' human rights.