Dixons Carphone, the company which owns Carphone Warehouse, has admitted a huge data breach involving 5.9 million payment cards and 1.2 million personal data records.
It is investigating the hacking attempt, which happened last year.
Dixons Carphone said it had no evidence that any of the cards had been used fraudulently following the breach.
There was "an attempt to compromise" 5.8 million credit and debit cards but only 105,000 cards without chip-and-pin protection had been leaked, it said.
The hackers had tried to gain access to one of the processing systems of Currys PC World and Dixons Travel stores, the firm said.
The 1.2 million personal data records accessed by the hackers consisted of non-financial information such as names, addresses and email addresses.
Carphone Warehouse said it had no evidence that the information had left its systems or resulted in any fraud, but it was contacting those affected to advise them.
Dixons Carphone chief executive Alex Baldock said it was "extremely disappointed" by the data breach and "sorry for any upset",
- Be on the look out for scam emails relating to this incident; many criminals will capitalise on breaches to send out viruses, or attempt to steal your personal information in fraudulent emails
- Check bank statements for any unusual activity - including regular, smaller amounts being withdrawn (which criminals do to try and go undetected)