The drive is being launched ahead of the introduction of the Government's Unclaimed Assets Scheme, the bill for which is currently going through Parliament.
Under the scheme, money that has been left in accounts for at least 15 years without any customer-initiated activity will be transferred to the Central Reclaim Fund and be reinvested in community projects, although people will never lose their right to reclaim their cash.
Halifax has so far handed back £17.5 million to customers that was languishing in forgotten accounts.
But it said it was still trying to trace the owners of four accounts with more than £100,000 in them, while it also has 25 dormant accounts with balances of more than £50,000.
Overall though, 70% of unused accounts have balances of less than £50, it said.
HSBC is writing to more than 12,000 customers who have not touched their accounts for 15 years, while it is trying to trace a further 5,000 with dormant accounts who are no longer at the address the bank has for them.
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