Banking giant HSBC said on Monday it has lost a computer disc containing details of nearly 400,000 customers.
The disc contains the names, dates of birth and insurance cover levels of 370,000 people who hold life assurance policies at the bank.
It does not contain addresses or bank account details, HSBC said.
"The data disk lost by HSBC contains no address or bank account details for any customer and would therefore be of very limited, if any, use to criminals," it said in a statement.
The data was password-protected, it added.
The disc went missing, it said, after being sent via Royal Mail courier from the group's offices in Southampton to the reinsurer Swiss Re at the beginning of February.
It is not clear at what stage of the journey the disc went missing.
"It has got lost between A and B," said HSBC spokesman James Thorpe. "There is no reason to suppose that the disk has fallen into the wrong hands."
Royal Mail said it had not been asked to look into the matter.
Normally such information is passed by secure wire link, but that was not working on the day the disc was sent.
HSBC said the reinsurer is carrying out a thorough search.
The bank has informed the City watchdog, the Financial Services Authority (FSA), which may carry out an investigation.
If it finds HSBC guilty of lax security, it could face a fine.
A spokesman for the FSA said it does not comment on the firms it regulates.
Norwich Union Life, one of the UK's largest life insurers, was fined 1.26 million pounds by the FSA last December for exposing its customers to the risk of fraud.
Weaknesses in its systems and controls allowed fraudsters to impersonate customers and obtain sensitive details from its call centres.
In some cases, they changed addresses and bank account details, helping them to then cash in policies totalling 3.3 million pounds last year.
