National Savings and Investments is urging Britain's 12m Ordinary Account customers to transfer their money now as the accounts will be shut in four weeks' time.
From 1 August, Ordinary Account customers will no longer be able to make deposits or withdrawals from their accounts, except to close accounts or transfer funds to other accounts.
The move follows NS&I's launch of the Easy Access Savings Account in January, which replaces the Ordinary Account.
Changing customer needs have forced NS&I to make the change, the executive agency says.
There are more than 11.9m Ordinary Accounts in the UK, however just 250,000 customers use these accounts regularly, with most not used for more than two years.
Hoping people currently not using their accounts will heed the warning, NS&I says Ordinary Account customers can also claim their lost or forgotten savings even if they no longer have their passbooks, or have not used their accounts for years.
It is estimated more than £200m may be lying dormant in accounts.
To close accounts or to transfer funds, customers can either obtain a form from their local post office, or call NS&I on 0845 366 6667.IFAonline
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