This security breach could have included names, addresses, dates of birth and bank details.
Up to 90,000 customers may also have had their encrypted credit card information accessed.
The affected division operates the OneStopPhoneShop.com, e2save.com and Mobiles.co.uk websites.
It also provides services to iD Mobile, TalkTalk Mobile, Talk Mobile and some Carphone Warehouse customers.
Dixons Carphone was formed last year by the merger of Carphone Warehouse and Dixons Retail.
Dixons Carphone operates in many European countries including Britain, Ireland and Germany.
Carphone Warehouse said it was informing all customers who may have been affected of the breach.
It will also advise affected individuals on how to reduce the risk of further consequences arising from the data leak.