Some families could be missing out on as much as £4,000 a year because they don't know they are eligible, according to a study by the consultancy Policy in Practice.
Pensioners are more likely not to apply for benefits on the grounds that they perceive benefit "handouts" to be shameful, say advisers in the study.
The study shared an example of how advisers helped a pensioner couple in Kent living on £280 a week secure an extra £222 a week in Pension Credit - £11,500 a year.
Another pensioner living in Coventry on £55 a week was found to be eligible for an extra £138 a week (£7,200 a year).?
An estimated 850,000 pensioner households fail to claim £1.8billion annually in Pension Credit.
Pension Credit is extra money to help people cover their costs if they're over state pension age and living on a low income.
They might be able to get it even if they have other income, savings or assets.
Pension Credit comes in two parts: Guarantee Credit and Savings Credit. It's separate from the state pension.
Mel Wright, money expert at Rest Less spoke exclusively with Express.co.uk about the help available for pensioners.
She said: "Those in their 60s, 70s and beyond are often on the lowest incomes, as they may not have had the opportunity to build up their retirement savings to provide them with enough to cover even the basics when they stop work.
?"Last winter we heard how many people in this age group were tragically forced to choose between eating or heating. Even though the weather is finally warming up, eye watering food costs mean that this summer many will still struggle to put food on the table.
?"Our members regularly tell us that they don't know how to go about claiming benefits, or that they've been refused payments that they thought they might be eligible for. The system is fiendishly complicated and often ends up letting down those who need it the most.
"It's crucial that more is done not only to raise awareness of some of the help that's available, but also to encourage people to claim, and to help them through the process from start to finish."
Pension Credit tops up weekly income to £201.05 if someone is single or their joint weekly income to £306.85 if they have a partner.
It is a significantly underclaimed DWP benefit, and is worth an average of £3,500 per year.
People might find they can claim extra money if they have other responsibilities and costs.
She urged Britons to check if they qualify as it can open the door to other financial support too.
She said: "For example, if you're eligible for Pension Credit, you should also qualify for housing benefit, council tax support, cost of living payments and, if you're aged over 75, a free TV license.
?"To think that so many thousands of people aren't benefitting from this help when they should be is tragic.
"If you think you might be eligible for support, there are several charities and organisations which can advise you on the benefits you might be entitled to and can help you submit a claim. "
These include Turn2us, which can assess one's eligibility for benefits through its Turn2us benefits calculator or by phone.
The site Entitledto.co.uk also has a free benefits calculator which you can use to see what one could qualify for.
Britons can also visit the Government website for information on how to claim.