Many older people could double their income if they claimed the benefits owed to them.
Up to £940 million is currently unclaimed in income support, according to Age Concern, which will urge older people to claim what is legally theirs during Your Rights Week, starting on 5 April.
Government figures released by the charity show that 770,000 older people eligible for income benefit are not claiming it, either because they are put off by the system's complexity or because they feel a sense of shame.
Hundreds of thousands of older people miss out on other benefits such as attendance allowance - which in some cases could enable them to double their income.
Initiatives planned for Your Rights Week include a benefits road show that will visit community centres and set up helpdesks for older people in supermarkets. A handbook listing all social security benefits underpins the campaign. The book, Your Rights - A Guide to Money Benefits for Older People, will be sold in branches of Age Concern and WHSmith.
Some MPs are collaborating with Age Concern to hold benefit rights surgeries during the week, said Katie Ghose, senior parliamentary officer for Age Concern.