Some £33million is being taken away from people's pensions as the Government ends the allowance for adult dependents.
Designed to help people with someone else relying on them, the benefit is worth £70 a week, but will stop being paid in April.
Steve Webb, Royal London policy director, said: "Under the old state pension system, people claiming a retirement pension could get a significant extra amount for a spouse who was financially dependent upon them."
Royal London launched a Freedom of Information request to find out just how many people would lose out - discovering 11,000 pensioners are still in receipt of the cash.
"It will come as a nasty shock to thousands of people to see their state pension cut by up to £70 per week," Webb said.
"Losing over £3,500 per year over night will make a material difference to the standard of living of those who are affected’."
The move comes as a result of the pensions Act 2007, which blocked people from applying for the credit after 2010.
But anyone already getting the extra cash by then was allowed to keep claiming it for as long as they were entitled.
That's stopping in April, seeing anyone with an "adult dependent" - generally a husband or wife under state pension age - lose the extra cash.
"It seems penny-pinching of the Government to take this money away when the addition is gradually working its way out of the system in any case," Webb said.
A DWP spokesperson told Mirror Online: “The ending of ADIs was part of a package of reforms introduced in 2010, which meant that overall more women received the full basic State Pension and more generous National Insurance credits for carers were introduced.
“After 6 April 2020, current ADI recipients may be eligible for a means tested benefit such as Universal Credit or Pension Credit. Those already in receipt of a means tested benefit should see no change to their income as the loss of the ADI will be offset by an increase in their means tested benefit.”
