Personal details of up to 2.4 million customers of the Carphone Warehouse could have been hacked following a cyber-attack on the mobile communications retailer.
Up to 90,000 customers could also have had their encrypted credit card details compromised the company said in a statement released today (8 August).
The retailer said: "On 5 August we discovered that the IT systems of a division of Carphone
Warehouse in the UK had been breached by a sophisticated cyber-attack.
"This division operates the websites OneStopPhoneShop.com, e2save.com and Mobiles.co.uk and provides a number of services to iD Mobile, TalkTalk Mobile, Talk Mobile, and to certain customers of Carphone Warehouse.
"We took immediate action to secure these systems and launched an investigation with a leading cyber security firm to determine exactly what data was affected. We have also put in place additional security measures to prevent further attacks."
The company, which joined forces with electrical retailer Dixons in a £3.8 billion merger last May, said that along with its partners it was in the process of contacting the customers who may have been affected by the breach to give them advice to reduce any risk.
"Currys and PCWorld and the vast majority of Carphone Warehouse customer data is held on separate systems and has not been accessed during this incident," the retailer said in a statement.
Sebastian James, group chief executive of Dixons Carphone, added: "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," he 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."
The company's investigation found that the data could have included names, addresses, dates of birth and bank details.
Last month Dixons Carphone reported a 20.6% jump in pre-tax profit to £381m in its maiden full year results since the merger between Dixons Retail and Carphone Warehouse.