The taxman says three and a half million people are due a refund, but two million will have to fork out for underpaid tax.
Over-75s' nest eggs 'shrinking'
The combination of high living costs due to inflation and low interest rates have eaten away at elderly people's savings
The long-term retired are having to "knock lumps" out of their nest eggs just to get by, pensioners' groups have warned, as a report found that saving pots for the over-75s have almost halved in the last two years.
In summer 2010, people in this age group typically had £22,500 put aside, but this figure has shrunk to £12,998, Aviva's latest Real Retirement report found.
The report also highlighted a "concerning" finding that almost a quarter (23%) of people over 75 have a credit card debt they do not repay in full on a monthly basis, despite the fact they are likely to have retired a decade earlier.
The typical person with credit card debt in this age group owes 128% of their monthly income, with £1,689 outstanding, the report found.
The National Association of Pension Funds (NAPF) said that the combination of high living costs due to inflation and low interest rates has eaten away at people's savings.
Joanne Segars, chief executive of the NAPF, said: "Inflation over the past few years has made life difficult for many pensioners, especially as low interest rates shrink any income from their savings.
"Many of those who are deep into their retirement are having to knock lumps out of their nest egg to get by. It's a very difficult and worrying position to be in, and many could see their savings run out.
"Sadly, the UK is sleepwalking into a crisis when it comes to its old age. We have to get more people saving into a workplace pension from as early as possible, and a simpler, more generous state pension will also help."
The Aviva report said that across the over 55 age group, savings had increased slightly from spring this year, from £14,198 to £15,756.
But the study said the increase was down to a jump in savings pots at retirement as people have used the tax-free sum from their annuity to boost their savings, followed by a decline in the assets of people who are older and have been forced to eat into their savings.
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