Martin Lewis has warned that around 750,000 Child Trust Funds remain unclaimed across Britain, with the average account now worth £2,200.
The money saving expert said many young people have no idea the accounts exist despite potentially having thousands of pounds waiting to be accessed.
Anyone born between September 1, 2002 and January 2, 2011 may have a Child Trust Fund (CTF), which was originally set up by Labour to encourage long-term savings for children.
The scheme saw eligible children receive vouchers worth up to £250, while those from lower income households were handed payments of up to £500.
Families were then able to add further money into the accounts over time, helping some funds grow significantly in value.
More than 6.3 million Child Trust Funds were opened during the scheme, with roughly 750,000 still unclaimed despite account holders now reaching adulthood and being able to access the cash.
Funds can only be withdrawn once the child turns 18, meaning many people are now eligible to claim money they may not even realise belongs to them.
Mr Lewis urged people to check whether they could be among those missing out on what could amount to a sizeable financial boost.
There is also good news for older account holders as reminders are now being sent to some people who have turned 21 to help them trace and access their accounts.
Martin Lewis warning as 750,000 unclaimed Child Trust Funds worth average £2,200 remain untouched
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Even those who have not received any correspondence are still able to track down their Child Trust Fund using a free HM Revenue and Customs (HMRC) service.
According to the Money Saving Expert website, people first need to identify which bank, building society or investment provider holds their account before they can access the money.
Those who already know where their Child Trust Fund is held can contact the provider directly to begin the process of reclaiming the funds.
For people who are unsure where their account is located, HMRC offers a free online tracing tool that identifies the provider holding the money.
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HMRC offers a free online tracing tool that identifies the provider holding the money
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Users can access the service using either a Government Gateway ID or a GOV.UK One Login account.
Applicants are then asked to provide personal details, or the details of their child if a parent or guardian is making the enquiry on their behalf.
HMRC said responses are typically issued within a few weeks once the request has been submitted.
Mr Lewis also warned people to remain vigilant against scammers attempting to exploit those searching for their Child Trust Fund.
In his latest newsletter, the financial journalist said: "Don't use details on there. Go and find the provider's details yourself and be careful of emails, calls and texts. This is a letter."
He warned people not to click links or call phone numbers included in suspicious messages claiming to relate to Child Trust Funds.
Instead, Mr Lewis advised account holders to independently search for official contact details to ensure they are dealing with the genuine provider.
He also pointed out that some firms charge customers to trace lost Child Trust Funds despite HMRC offering the same service completely free of charge.
Labour closed the Child Trust Fund scheme in 2011 and replaced it with Junior ISAs, although existing accounts have remained active and continue earning interest or investment returns until withdrawn.