A division of the Co-operative Group, which organises funeral planning and wills, has admitted that data on 83,000 of its customers was accidentally posted online.
A spokesperson for Co-operative Life Planning (CLP) explained that the information was lost by a third-party company used by CLP, but stressed that the data was not of a sensitive nature.
"As a result of an error at a company which provides technical support services to CLP, the security of some data was lowered. The data related only to funeral planning products, not wills, held by Co-operative," the spokesperson said.
"The data is classified as non-sensitive under the Data Protection Act. It did not contain any bank account details, National Insurance numbers, health details, telephone numbers or emails."
Nevertheless, the group said that a full investigation is underway to find out how the incident occurred, and that the Information Commissioner's Office (ICO) has been notified.
Co-operative Group has also written to the 83,000 affected customers to inform them of the breach.
The ICO confirmed it had been contacted about the breach and said it would be investigating.
"We have recently been informed of a possible data breach which may involve the Co-op," they said.
"We will be making enquiries into the circumstances of the alleged breach of the Data Protection Act before deciding what action, if any, needs to be taken."
The breach marks yet another data loss incident for the ICO to deal with after Leicester City Council admitted that a USB stick containing important information on 8,000 at-risk citizens was mislaid.
It also comes on the same day that BP admitted to the loss of an unencrypted laptop containing the details of 13,000 people who had made compensation claims after being affected by the Deepwater Horizon oil spill last year.
Found by calculating the strength of the material deep inside the crust of neutron stars
Can highlight in real-time the relevant regions of an image being described
Double legal trouble for Musk as he also faces civil lawsuit over renewed British pot-holer 'paedo' claims
Battery development could help boost performance of smartphones