Around 827,000 young adults have a matured Child Trust Fund account waiting to be claimed, which could be worth £2,310 on average, according to HM Revenue and Customs (HMRC).
The revenue body is encouraging people to track down their savings accounts.
Nearly three million accounts held by 18 to 24-year-olds have either been claimed or transferred into an Isa.
Child Trust Funds are long-term, tax-free savings accounts set up for children born between September 1 2002 and January 2 2011, with an initial government deposit of at least £250.
The Child Trust Fund scheme closed in January 2011 and was replaced with Junior Isas.
The savings accounts are not held by government, but are with banks, building societies or other savings providers.
Young people can take control of their account at 16 and when the account matures at 18, they can decide what they want to do with it.
They could withdraw it to spend, leave it where it is or reinvest it.
Nearly 500,000 young people used a gov.uk locator tool to find their Child Trust Fund in the nine months to August this year, HMRC said.
If young people, or their parents or guardians, know who their Child Trust Fund provider is, they can contact them directly.
A Child Trust Fund Taskforce has been established to bring together Government and industry to help reduce the number of unclaimed accounts.
Economic Secretary to the Treasury Lucy Rigby, who leads the taskforce, said: “The oldest matured accounts are now more than six years old and some young people may not know they have savings waiting for them.
“The taskforce has been set up to help those young people who don’t know about their savings.
“If you’re unsure where your account is, you can find it, quickly, easily and for free using the gov.uk locator tool.”
The savings accounts are not held by government but are with banks, building societies or other saving providers.
