A "coding issue" on the Dabs.com website resulted in some customers getting access to other users' personal details.
The issue came to light after a PC Pro reader logged into his account with the BT-owned retailer, only to find his account details had been merged with that of another customer.
After logging in to his Dabs.com account at the weekend, he was redirected to BT Business Direct, another online retailer in the same group.
There he found the name, address, telephone number and 12-month order history of another named customer merged in with his own details.
"I was very concerned and upset about this," the reader, who wishes to remain anonymous, told PC Pro. "But when I tried to phone Dabs.com [to complain] I was referred back to the website."
A BT spokesman told us: "Over the weekend we moved customer details to an upgraded platform. Due to a coding issue, a tiny number of customers, for a limited time only, were able to see the order and address details of another customer.
"At no time were these customers able to see the financial details of other customers. We apologise for this issue, which was quickly rectified."
BT purchased the Bolton-based Dabs.com for around £30m in 2006.
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