The company behind Curry’s PC World and Carphone Warehouse identified the breach last month and has been reviewing its systems security since.

It said it had found evidence that some data had left its systems, but that records do not contain payment card or bank account details. It found no evidence of fraud.

The hackers also accessed 5.9 million cards, but 5.8 million had chip and pin protection.

The retailer has brought in cyber security experts and added extra security measures to its systems.

Dixons Carphone chief executive, Alex Baldock, said: “As a precaution, we’re now also contacting all our customers to apologise and advise on the steps they can take to protect themselves.

“Again, we’re disappointed in having fallen short here, and very sorry for any distress we’ve caused our customers. I want to assure them we remain fully committed to making their personal data safe with us.”

Alex Neill, Which? managing director of home products and services, said: “Dixons Carphone customers will be alarmed to hear about this massive data breach and will be asking why it has taken so long for the company to uncover the extent of its security failure. It is now critical that the company moves quickly to ensure those affected get clear information about what has happened and what steps they should take to protect themselves.

“Anyone concerned they could be at risk of fraud should consider changing their online passwords, monitor bank and other online accounts and be wary of emails regarding the breach as scammers may try and take advantage of it.”