Thousands of valuable items stored in bank safety deposit boxes cannot be traced to their owners, a BBC investigation has been told.
Sources at Lloyds, RBS and Barclays told BBC Radio 4's You and Yours that branches have been running disorganised "safe custody" systems.
Changes to the services have also seen some customers unable to retrieve items, the sources said.
The banks said they were committed to storing customers' items safely.
Gillian Robinson, from Dorset, lost an estimated £10,000 of family heirlooms in 2013 when Lloyds Bank failed to locate them. She has since been reimbursed.
Gold jewellery from India which her parents had received as traditional wedding gifts was stored with Lloyds when they came to the UK.
The box included a necklace handed down by her great-grandmother which she described as "priceless".
Mrs Robinson said: "I got a call from the bank, saying they were transporting everything from their vaults up north to a safety deposit area, and then they discovered they couldn't find mine.
"Inside that box was my mother's family jewellery, jewellery given for weddings. It's all gone.
"The bank asked whether my jewellery was insured and I said 'No, that's why it was in the bank - I am paying for it to be safe'."
One source at Lloyds Banking Group said it discovered the extent of the problem when they undertook an audit of UK branches in 2014.
The source said they identified 100,000 items which had to be traced back to their owners.
Of those, the source claims 10,000 items were classed as untraceable and ranged from jewellery, gold and family heirlooms, to important documents such as passports, property deeds and wills.
Last year the Financial Ombudsman received 65 complaints about "safe custody" services.
In recent years most high street banks have stopped offering them. The changing physical layout of many banks and the lack of vaults mean fewer of them have the facilities and many people are choosing to have home safes instead.
How do you protect your valuables?
- Deposit boxes are used by many people to protect their assets or store valuable family heirlooms
- It is a good idea to photograph anything you put in a deposit box
- If your bank does not ask you to sign a log when you visit the box, ask why
- If you die while renting a box, the executor of your estate, your heir or any joint account holders should be able to access the box - however, this is dependent on them knowing that it exists
- When rental fees for the box stop being paid, the bank or provider can claim the contents
David Ellerton, from the Safety Deposit Association, said problems often date back to times before computers when items were recorded using ledgers.
"We do get weekly calls on the subject because the banks don't seem to find peoples' belongings but… unless people can prove what was in the storage, the ombudsman has been finding in favour of the banks," he said.
A spokesman for Lloyds Banking Group said the safety of customers' information and property was a priority and it was investing in the transition from paper records to a central online database.
RBS said it did not have a large-scale problem, but on occasion they had been unable to locate items. They added problems can occur if customers failed to update the bank with contact details.
A spokesperson from Barclays said: "Following the closure of the Barclaysafe Premium Service in 2013, an extensive programme of activity was undertaken to contact customers and return their items.
"Any items not retrieved during this time were then moved to centralised secure storage."