As many as 2 million CeX customers may have had their personal information stolen after the electronics retailer suffered a cyber attack.
The company - a second hand goods chain specialising in tech, computing and video games and has a shop in Sheffield city centre - also runs the webuy.com website which has been subject to an attack.
Data compromised by the hackers includes first names, surnames, addresses, email addresses and phone numbers of registered CeX customers.
As a precautionary measure the company is emailing up to 2 million of their registered website customers who could potentially be affected and asking them to change their passwords immediately.
Customers, who do not receive an email, are not affected.
Managing director David Mullins said: "This was a sophisticated breach of security and we are working closely with the relevant authorities to help establish who was responsible.
"Our cyber security specialists have already put in place additional advanced measures to fix the problem and prevent this from happening again."