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.
Chancellor George Osborne said in his inaugural 2010 budget the government would increase state pensions in line with the highest of CPI, average earnings or 2.5%.
Traditionally, if inflation is used to index the pensions, the inflation figure in September of the current year is used as the basis for the state pension increase in the following April.
However, Dot Gibson, general secretary of the NPC, said pensions minister Steve Webb has refused to confirm which month's inflation the government will use as September's CPI stood at a record high of 5.2%.
"I do not understand why the minister cannot simply tell us 5.2% is the figure they will use to raise our pensions next year.
"It appears the government has made a promise which it does not intend to keep," she said.
CPI has increased dramatically this year, rising from 4.5% in April to 5.2% in September.
Using April's inflation rate would boost a single person's basic state pension by £239.03 per year, but increasing it by September's rate would mean an uprating of £276.21.
The rate that will be used to increase pensions is normally confirmed in the Social Security Benefits Uprating Order, announced in Parliament in December.
Gibson said Webb would only confirm the state pension increase will be in Osborne's autumn statement on 29 November, but would not confirm how it would be set.
A spokesperson for the Deparment for Work and Pensions (DWP) did not confirm which month's CPI would be used.
The spokesperson said: "Next year's benefit rates will be announced to Parliament later this autmn."
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