Dixons Carphone has admitted a huge data breach involving 5.9 million payment cards and 1.2 million personal data records.
It has begun investigating the unauthorised access of data.
It said 5.8 million of the credit and debit cards had chip-and-pin protection and that pin codes had not been accessed.
As a result, about 105,000 non-EU cards, which were not chip-and-pin, had been compromised, it said.
Dixons Carphone said it had no evidence that any of the cards had been used fraudulently following the breach.