Marks and Spencer had to suspend its website for two hours last night after customers were able to see other people's details.
Managers say they were not hacked and no one's financial information was compromised by what they are calling an "internal technical problem".
Shoppers have been posting on Facebook that they could see names, addresses, and previous orders of other people.
The company have said they were experiencing technical difficulties and were working on the problem.
A spokesperson said: "We're currently experiencing some technical difficulties and have temporarily suspended our website whilst we investigate this thoroughly. We're working hard to fix this and will update as soon as we can."