Dixons Carphone Plc said a cyberattack affected nearly 6 million payment cards as hackers sought unauthorized access to customers' personal data.
'The protection of our data has to be at the heart of our business, and we've fallen short here.
However the hackers were able to access the personal data records of 1.2 million customers, with details such as names, addresses and email addresses all leaked.
However, it was quick to add that 5.8 million of these cards had chip and PIN protection, and that the data stolen did not include pin codes, card verification values (CVV) or authentication data - making it more hard for the hackers to monetize the breached data.
"An incident involving Dixons Carphone has been reported to us and we are liaising with the National Cyber Security Centre, the Financial Conduct Authority and other relevant agencies to ascertain the details and impact on customers", the agency said in a statement. However, 105,000 credit cards that did.
Dixons Carphone said relevant card companies had been notified, but added that there was no evidence of fraud on the cards as a result of the incident.
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The group added it did not believe the personal data accessed had left the group's systems, but was advising those affected on protective steps they should take.
Shares in Dixons Carphone dropped 4% in London.
Dixons Carphone said that action had been taken to close off the access, and there was no evidence to suggest it was continuing. It said since the 2015 attack it had worked extensively with cyber security experts to upgrade its security systems.
It said the data accessed did not contain Pin codes, card verification values (CVV) or any authentication data allowing cardholder identification or a purchase to be made.
Given the small number of affected cards and the fact that personal data did not leave the network, it's unlikely the firm will be in for a major GDPR fine, unless it emerges that the hackers took advantage of serious deficiencies in the firm's cyber-defenses.
Baldock joined Dixons Carphone in April and last month the group warned on profits and said it would have to close shops, wiping more than 500 million pounds off its stock market value.