Marks & Spencer was forced to suspend its website after customers were able to see other people's details when they logged in to their accounts.
Customers posted messages on the high street chain's Facebook page to say they could see other people's orders and payment details when they logged into their accounts.
The firm said no customer's details were compromised by the "technical difficulties".
Customers may have been able to see the last four digits of another person's payment card "for a brief moment", an M&S spokesperson said, adding that there was no security risk as details were encrypted.
Facebook users said they could see various personal details online as a result of the "technical difficulties".
Konstantinos Vlassis posted:
Shopper Debbie Dilks called the issues "appalling".
Fellow customer Vanessa Frost also vented her anger online.
An M&S spokesperson said: "Due to a technical issue we temporarily suspended our website earlier this evening.
"This allowed us to thoroughly investigate and resolve the issue and quickly restore service for our customers.
"We apologise to customers for any inconvenience caused."
Have you been affected? You can share your experience at firstname.lastname@example.org or by commenting on Facebook