Up to 2.4m Carphone Warehouse customers may have had personal details accessed by hackers in a massive cyber attack.
The retailer said that encrypted credit card details of up to 90,000 customers may have been part of data taken on 5 August.
The IT systems division of Carphone Warehouse in the UK operates websites OneStopPhoneShop.com, e2save.com and Mobiles.co.uk and provides a number of services to iD Mobile, TalkTalk Mobile, and to some customers of Carphone Warehouse.
Dixons Carphone chief executive Sebastian James 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.
The company has taken action to secure the affected systems and launched an investigation with a "leading cyber security firm" into determining the exact extent of the breach.
Customers who may have been affected will be given advice by Carphone Warehouse on how to reduce risk.
Currys, PCWorld and the majority of Carphone Warehouse customers are not affected as their details are held on different systems.