Savings giant NS&I is undergoing a programme to reunite people with their cash, with up to £476 million in deposits being affected.
Concerns have been raised over bereaved families struggling to access their money.
Pensions minister Torsten Bell told the House of Commons that NS&I notified the Treasury of an operational failure to trace accounts comprehensively of some customers who had died.
He said: “The result of this failure is that not all savings were identified by NS&I and paid to the beneficiaries of their estates as they should have been.
“Specifically, processes failed to comprehensively trace some customer holdings where those were spread across multiple profiles or systems.”
Mr Bell said: “Bereaved families whose loved ones held accounts with NS&I will rightly be anxious about this news.”
He added: “Since being notified, the Treasury has ensured external advisers including EY and legal experts have been engaged to identify the scale of these errors.
“Through this work, NS&I have reviewed over 34 million customer records.
“That work is ongoing, but it points to a maximum of around 37,500 customers with up to £476 million in deposits being affected.
“Three-quarters of cases relate to the period between 2008 and 2025.
“This number is likely to fall in future but while it represents less than 0.2% of NS&I’s customers, it is still far too many.”
NS&I offers a range of savings and investments to more than 24 million customers, including more than 22 million Premium Bonds holders.
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