Millions of teenagers are sitting on a combined £394m windfall, but most have no idea it is there or how to get it.
The National Audit Office (NAO) said that a huge proportion of child trust funds (CTFs) remain untouched for a year or more after their owners turned 18.
The spending watchdog said some 145,000 young people have failed to claim the money which was invested for them in child trust funds.
A CTF is a savings account for children born between 1 September 2002 and 2 January 2011, which they can access at the age of 18.
The government paid more than £2bn into CTFs for 6.3 million children born during the qualifying period. Most children received around £250 each from the government at the time their CTF was started, while those from low-income families or in local authority care received an additional £250.
By April 2021, around 320,000 CTFs had matured in the seven months since the first CTF account holders turned 18 in September 2020.
Of these, 175,000 (55%) had been claimed by the account holders and the accounts closed, and 145,000 remain unclaimed.
Some £394m was, by April 2021, yet to be claimed in matured CTFs belonging to young adults who had reached the age of 18, the NAO said.
This means these teenagers have £1,900 in trust funds they don’t know about.
Gareth Davies, the head of the NAO, said: "At a time of economic hardship for millions of people across the country, it is important the government does enough to make sure young people are aware of, and can access, their child trust funds."
The NAO said HM Revenue and Customs intends to incorporate CTFs into a communications campaign in 2023.
Public Accounts Committee chair Meg Hillier said people need to be proactively helped to be reunited with their funds, adding that, in a cost of living crisis, the money could be “a vital lifeline to young people, particularly those from low-income backgrounds”.