Around £2.8 billion of pension-age benefits went unclaimed in 2019/20, figures from the Department for Work and Pensions revealed.
Estimates of take-up suggest seven out of ten of those entitled to pension credit claimed the benefit, taking 77% of all that could have been claimed.
Similarly, eight out of ten pensioners entitled to housing benefit took advantage of it, claiming 86% of the total amount that could be claimed.
That means up to £1.7bn of available pension credit and £1.1bn of housing benefit for pensioners went unclaimed.
Pension credit is available to people over state pension age and on a low income, while housing benefit is available to those above the state pension age and are unemployed, on a low income or claiming benefits.
Richard Murphy, political economist, said the Government should be encouraging people to claim "the benefits they are due".
He added:
"This is £2.8 billion unpaid that could make a massive difference to people's lives."
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