Millions of people currently entitled to the state second pension will be worse off as a result of the government's pension changes, the TUC has claimed.
A TUC report says that anyone with a long working history is likely to lose out, by as much as £2,000 a year, the BBC reports.
The second state pension will be abolished when the new single tier pension begins in April 2016.
But the government said the changes will make most people better off.
Around 20 million Britons are currently part of the state second pension scheme, introduced ten years ago to boost pension levels for low earners.
The TUC report suggests that the "vast majority" of them will get less money when they retire.
"Many low and middle-income private sector workers, particularly those several decades away from retirement, could be thousands of pounds a year worse off in retirement," said Frances O'Grady, the TUC general secretary.
The TUC supports the principle of the single tier pension, but wants it to be raised from the current notional level of £144 a week.
The study claims that anyone on a median income of £26,000 a year, and who has a full employment record, will be worse off as soon as the new pension is introduced.
Such a person retiring in 2030 would receive £1,500 a year less than under the current system.
Someone retiring ten years after that would be £2,000 a year worse off.
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