Figures from HMRC show that 760,000 matured Child Trust Funds, worth an average of £2,000 each, remain unclaimed.
The FCA has urged young adults and parents to check whether they have a forgotten Child Trust Fund.
The regulator pointed out that hundreds of thousands of young people could be missing out on money that could go towards new laptops, textbooks, rent deposits or the everyday costs of student life – or setting them up with savings for their future.
The FCA warned consumer to be wary of companies, including those advertising on social media, which offer to 'find' and claim Child Trust Funds on people's behalf – often for a hefty fee or a cut of the final payout.
The regulator said it has seen cases where customers were charged £400 to locate the account and even seen some firms charging a monthly subscription for a one-off tracing service. But missing funds can be traced for free without losing a penny to a middleman.
Chris Knight, director of insurance at the FCA, said: "You don't need to pay someone else to claim what's rightfully yours – tracing and accessing your own Child Trust Fund costs nothing, so think twice about handing over a chunk of your savings to a claims firm for a job you can do yourself."
Young adults can trace a lost Child Trust Fund for free using HMRC's online tool at GOV.UK.
The FCA is also launching a review into Child Trust Funds. It will look at issues including cases where young adults cannot be contacted when they turn 18 and risk losing touch with their savings altogether.
The review will also look at how firms are ensuring Child Trust Fund customers receive fair value under the Consumer Duty, and whether there are barriers to vulnerable young adults accessing their money. The review will report next year.
• Child Trust Funds were available to children born between 1 September 2002 and 2 January 2011, with the scheme now closed to new applicants. Around 6.3m accounts were opened.