Dixons Carphone 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 began in July 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" 5.9 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.
Dixons Carphone shares fell more than 3% in early trading.
Analysis by BBC technology correspondent Rory Cellan-Jones on the face of it, this is a very serious incident.
Usually, when companies report a data breach they are very quick to reassure us that while names, email addresses and login may have been accessed, no payment information has been released.
This is not the case here with Dixons admitting that hackers got access to records of nearly six million payment cards.
The good news is that nearly all of them were protected by good old chip and pin - and there is no evidence of any fraud relating to the 100,000 non-European cards which didn't have that protection.
But there are still questions for Dixons Carphone to answer.
Dixons insists that it only discovered this latest hack a week ago and it has no connection with any previous incident.
But the UK Information Commissioner's Office (ICO) which fined Carphone Warehouse £500,000 for the 2015 breach will now be looking very closely at this latest failing of the merged companies.
Luckily for Dixons, the incident happened before the new GDPR rules, which promise much bigger fines, came into force.
The 1.2 million personal data records accessed by the hackers consisted of non-financial information such as names, addresses and email addresses. The spokesman also stated that "Anyone concerned about lost data and how it may be used should follow the advice of Action Fraud."
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. It added that it had brought in leading cyber-experts and added extra security measures to its systems. Dixons Carphone chief executive Alex Baldock said he was "extremely disappointed" by the data breach and "sorry for any upset".