More than one million pensioners living with long-term health conditions or disabilities could be missing out on a benefit worth up to £458.40 every month.
According to estimates from MoneySavingExpert and Policy in Practice, around 1.1 million people above state pension age are not claiming Attendance Allowance despite being eligible.
The organisations estimate that around £5.2billion in support goes unclaimed every year.
MoneySavingExpert, founded by Martin Lewis, described Attendance Allowance as one of the most under-claimed benefits available.
The consumer website said the support remains "still massively under-claimed" despite publishing a detailed guide to the benefit two years prior.
More than 1.7 million people across Great Britain currently receive Attendance Allowance, which can add up to £5,959 a year for successful claimants.
Attendance Allowance is designed to help older people meet additional costs linked to physical or mental illnesses and disabilities that make daily living more difficult.
The benefit is paid at two different rates depending on the level of support required.
DWP Attendance Allowance warning as 1.1 million pensioners miss benefit worth £458 a month
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People needing help either during the day or at night can receive £76.70 a week.
Those requiring support both day and night, or people who are terminally ill, may qualify for the higher weekly rate of £114.60.
Because the payments are made every four weeks, claimants receive either £306.80 or £458.40 per payment period.
Attendance Allowance is tax-free and is not means-tested, meaning savings and income do not affect eligibility.
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Recipients can spend the money however they choose, including on transport costs, household bills or support around the home such as cleaners or gardeners.
People who need supervision or support during the day or night are encouraged to consider applying even if they are not currently receiving formal care.
This includes those who struggle with daily personal care tasks such as getting dressed, washing, eating or using the toilet.
People who require help staying safe or who find everyday activities painful, time-consuming or physically difficult may also qualify.
The benefit is not restricted to physical conditions alone.
Those with mental health conditions, learning difficulties or sensory impairments, including deafness or visual impairment may also be eligible if they need regular support.
Eligibility is based on the level of help a person needs rather than the care they currently receive.
Claimants can still qualify even if they have substantial savings or remain in employment.
Attendance Allowance also does not affect state pension payments.
Receiving the benefit could increase entitlement to other forms of support including Pension Credit, Housing Benefit and Council Tax reductions.
The application process involves completing a lengthy form, although assistance is available through local Citizens Advice services.
People approaching state pension age may also wish to consider claiming Personal Independence Payment before reaching pension age as it could provide higher levels of support in some cases.