300,000 missing out on unemployment benefits
The Resolution Foundation said huge numbers were not claiming unemployment benefits worth at least £73 a week.
The think tank said the group of "forgotten unemployed" were mainly older people, especially women aged 55 to 64, and younger men.
Most were jobless, but a significant minority were in work and entitled to claim Universal Credit or Jobseeker's Allowance.
Many don't claim benefits if they were living with a partner or parents, said the Foundation, which urged the Government to do more to help those on the "margins" of the labour market.
David Finch, senior economic analyst at the Resolution Foundation, said: "Over the last 20 years, a growing number of unemployed people are not claiming unemployment benefits.
"Policy makers have generally been pretty relaxed about this gap, assuming that is largely due to people finding new work very quickly, or having other sources of financial support at home.
"But while there are good reasons for some people not to claim benefits, there are also around 300,000 forgotten unemployed people who are falling through the cracks and not getting the financial support that they need and are entitled to."
A Department for Work and Pensions spokesman said: "Anyone who believes they're entitled to out of work benefits should contact Jobcentre Plus - online, over the phone or by visiting their local branch.
"Our dedicated advisers and work coaches are on hand to help people claim what they are entitled to, and can signpost to other support options available."