Lloyds Banking Group discovered a vast trove of old wills it could not match to customers, leading to hundreds of families distributing assets to the wrong beneficiaries.
The 9,000 wills had been stored as part of a "safe custody" service that was closed to new customers in 2011, according to internal documents seen by the Financial Times.
The discovery potentially creates a difficult situation for families and executors who may have administered a deceased's estate using what turned out to be the wrong will, not knowing that the right one was stored at Lloyds.
The bank said it is investigating each case individually and, where appropriate, making contact with representative of these estates. It added that it will ensure that those affected are "fully compensated" and no funds already distributed will be clawed back.
In 90% of cases wills were superseded by a newer version, while in others copies of wills were held elsewhere, or had been settled through the intestacy process but in line with the deceased person's wishes.
Despite that, it estimates that in hundreds of cases, assets were incorrectly distributed.
Michael Culver, chairman of Solicitors for the Elderly, recommends that the families affected make a complaint to the Financial Ombudsman and make a counter claim against Lloyds directly.
The wills were found among some 190,000 valuable papers stored by Lloyds' safe custody service. The bank has now centralised items and digitalised papers it still holds under the safe custody scheme.