Thousands of former Dairy Crest milk deliverers will have to make do with smaller pension increases.
They are among 15,000 members of the company’s pension scheme, who have been sent letters giving details of the cuts.
Until now the pensions, based on an employee’s final salary, were increased each year in line with the Retail Prices Index (RPI) inflation measure.
In future, the increases will follow the Consumer Prices Index (CPI).
That measure tends to be about one percentage point less.
As things stand, the scheme’s £1bn worth of investments will not be enough to cover its future pension bills.
But Dairy Crest will report an exceptional gain of £125m in its accounts because of the move.
And it will cut its cash contributions by £12m a year for two years.
The chair of the pension trustees, Vicky Paramour, told the BBC that the scheme’s rules allowed the switch.
She said the company had made commitments to maintain subsequent contributions and that the changes “will put the fund on a more secure footing for the future”.
About 8,400 of the members are already drawing their pensions. The remainder are still working or waiting to reach pension age.