Electronics retailer Dixons Carphone has admitted to a customer data breach in which hackers gained access to 5.9 million payment cards and 1.2 million personal data records.
Those behind the hacking, which began in July last year, had tried to access one of the processing systems of Currys PC World and Dixons Travel stores.
It is currently being investigated.
Dixons Carphone said it had no evidence of any card being used fraudulently after the breach as nearly all of them were protected by their chip and pin, meaning the details of 105,000 cards without chip-and-pin protection were at risk of being leaked.
The company also said it first knew of the hack attempt a week ago and that it has no connection with any previous incidents.
Since the announcement, Dixons Carphone shares decreased more than 3% in early trading.