The personal details of up to 2.4 million customers may have been accessed after a division of Carphone Warehouse was hit by a cyber attack, the mobile phone retailer has said.
An investigation carried out by the company found that the data could have included names, addresses, dates of birth and bank details.
The encrypted credit card information of up to 90,000 customers may also have been accessed, a statement released by the firm warned.
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."