Marks & Spencer was forced to take down its site for two hours on Tuesday after a fault allowed users to see other customers' details.
The retailer said the fault was a result of a technical, in-house error, rather than a cyber attack. Customers were able to see other customers' details when logging into their account.
Credit card details were not among the information leaked in the fault. Instead, names, dates of birth, contact information and previous orders were visible.
"Due to a technical issue, we temporarily suspended our website yesterday evening," an M&S spokesman said.
"This allowed us to thoroughly investigate and resolve the issue and quickly restore service for our customers."
Marks & Spencer is yet to reveal how many customers' information was affected by the leak.