Dixons Carphone Plc said a cyberattack affected nearly 6 million payment cards as hackers sought unauthorized access to customers' personal data.
The Dixons Carphone share price fell by 5.5% within minutes of major hack announcement.
Last month, the retailer forecast that earnings this year will slump about 21 percent as it closes mobile-phone stores in a contracting United Kingdom household-electronics market.
"We are extremely disappointed for any upset this may cause".
Dixons Carphone said that action had been taken to close off the access, and there was no evidence to suggest it was continuing.
Dixons Carphone (DSITF) said Wednesday that it had discovered an attempt to compromise card processing systems at its Currys PC World and Dixons Travel stores.
According to the company, the vast majority of the cards (5.8M) were protected by chip-and-PIN technology - and it says the data accessed in respect of these cards contains "neither pin codes, card verification values (CVV) nor any authentication data enabling cardholder identification or a purchase to be made".
Dixons Carphone said that "unauthorised access" of data held by the company had prompted an investigation, the hiring of external security experts and efforts to shore up its security defences. "We have no evidence of any fraud on these cards as a result of this incident", it writes.
In addition, Dixons Carphone said 1.2 million personal data records were hacked. It said there was no evidence of fraud here either. It has informed police, regulators at the Information Commissioner's Office and the Financial Conduct Authority.
The ICO said it was liaising with the National Cyber Security Centre, the FCA and other agencies to determine the impact on customers. "As a multinational organisation, Dixons Carphone would have been well aware of the Target breach". It paid 320,000 pounds as there was a 20 percent reduction for early payment.
It comes after telecoms firm TalkTalk was hit by a major cyber attack in October 2015, which saw the personal data of almost 160,000 people accessed by hackers and left the firm facing a record £400,000 fine for security failings.
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