Benefits worth billions of pounds - ranging from pension credit to jobseeker's allowance - go unclaimed each year, figures show.
Up to £2.8bn in pension credit, used to top-up pensioners' weekly income, was not taken up, the Department for Work and Pensions (DWP) said.
A charity has called on the government to make people more aware of what they are entitled to.
Age UK described the total lost to pensioners as "staggering".
The DWP has outlined data on the various income-related benefits that were not claimed. The latest figures, which refer to 2009-10, include:
- Up to 620,000 people failing to claim up to £2bn in income support, and employment and support allowance
- Up to 1.6 million people failing to claim up to £2.8bn in pension credit
- Up to 1.1 million people not claiming up to £3.1bn in housing benefit
- Up to 3.2 million people missing out on up to £2.4bn in council tax benefit
- Up to 610,000 people failing to claim up to £1.95bn in jobseeker's allowance
Charities have picked out the losses suffered by the elderly.
"It is very disappointing that there has been no progress in older people not claiming the benefits they are owed," said Michelle Mitchell, charity director general of Age UK.
"There are still 1.8 million people in later life living in poverty and claiming pension credit can make a huge difference to someone's income and quality of life.
"The government needs to start an awareness programme and move more towards a system where the DWP pay entitlements rather than an individual having to work their way through the benefits maze."
Pensions minister Steve Webb said: "The goal is for people to retire on decent income, which is why we are bringing in automatic enrolment and have consulted on proposals to reform the system to provide a simple state pension set above the level of the basic means test".