Dixons Carphone has said a huge data breach that took place last year involved 10 million customers, up from its original estimate of 1.2 million.
Dixons has been investigating the hack since it first revealed it in June.
It said personal information, names, addresses and email addresses may have been accessed last year, but not bank details and it found no evidence that fraud had resulted from the breach.
Dixons said it was "very sorry for any distress" caused.
The hackers also got access to records of 5.9 million payments cards, but nearly all of those were protected by the chip and pin system.
Dixons said it had been working with leading cyber security experts and had put in further security measures to safeguard customer information.
"Since our data security review uncovered last year's breach, we've been working around the clock to put it right," said Dixons Carphone chief executive Alex Baldock.
"That's included closing off the unauthorised access, adding new security measures and launching an immediate investigation, which has allowed us to build a fuller understanding of the incident that we're updating on today.
"As a precaution, we're now also contacting all our customers to apologise and advise on the steps they can take to protect themselves."