Up to 850,000 families who were entitled to receive Pension Credit did not claim the benefit in the financial year 2019 to 2020, according to Government figures.
The Department for Work and Pensions (DWP) said up to £1.7 billion of available Pension Credit went unclaimed.
On average, this amounted to around £1,900 per year for each family entitled to receive Pension Credit who did not claim the benefit.
Pension Credit gives people extra money to help with living costs if they are over state pension age and on a low income.
An estimated seven out of 10 of those entitled to Pension Credit did claim.
Helen Morrissey, senior pensions and retirement analyst at Hargreaves Lansdown, said: “Pension Credit take-up continues to inch up but there are still up to 850,000 pensioner households missing out on this vital benefit.
“Pension Credit boosts the income of the poorest pensioners and acts as a valuable gateway to other benefits such as help with heating bills and NHS treatment. If you are aged over 75 you are also entitled to a free TV licence.”
Up to 260,000 thousand pensioners who were entitled to receive Housing Benefit did not claim the benefit, according to the figures. On average, this amounted to around £4,000 per year going unclaimed.
Ms Morrissey said: “These are enormous sums of money that could really boost people’s income in retirement, and it is vitally important that those who think they may be entitled to support check to see if they are entitled.”
People can find out more about making a claim on the gov.uk website.
Sir Steve Webb, a former Liberal Democrat pensions minister who is now a partner at consultants LCP (Lane Clark & Peacock), said: “With the cost-of-living crunch likely to hit pensioners in particular, it is more important than ever that older people claim all the benefits to which they are entitled.
“DWP should be much more proactive in using their own records to identify people who may be entitled and have not yet claimed and encourage them to put in a claim.
“Claiming Pension Credit can be a ‘passport’ to a wide range of help with other costs such as energy bills, council tax or TV licences, so even if someone is only entitled to a few pounds of Pension Credit, the knock-on benefits can be substantial.
“A claim can be made over the phone and is a relatively simple process and I would encourage anyone who thinks they may be eligible to make a call to find out.”
A DWP spokesperson said: “We want to ensure pensioners receive all the support to which they are entitled and it is encouraging to see that take-up of Pension Credit increased in 2019/20, with the number of new claims 30% higher in 2021 compared to 2019.
“We continue to work with stakeholders and others to encourage those eligible for Pension Credit, and the wide range of other benefits it can provide, to make a claim.”