The details of up to 2.4 million Carphone Warehouse customers may have been accessed after the firm was targeted by hackers.
A statement from parent firm Dixons Carphone said names, addresses, dates of birth and bank details may be among the information in the security breach.
It said encrypted credit card data of up to 90,000 customers may also have been accessed in the hack on 5 August.
The division affected operates the websites OneStopPhoneShop.com, e2save.com and Mobiles.co.uk.
All of the sites were down on Saturday afternoon.
The attacked division also provides a number of services to iD Mobile, TalkTalk Mobile, Talk Mobile, and to certain customers of Carphone Warehouse, the statement said.
The company said the vast majority of Carphone Warehouse customer data and that of Currys and PCWorld is held on separate systems and was not accessed during the attack.
It described the cyber attack as "sophisticated" and said a leading cyber security firm had been drafted in to work out what information had been compromised.
Sebastian James, Group Chief Executive of Dixons Carphone, said: "We take the security of customer data extremely seriously, and we are very sorry that people have been affected by this attack on our systems.
"We are, of course, informing anyone that may have been affected, and have put in place additional security measures."
