People in Sheffield could be among 10 million customers affected by a data breach at the Carphone Warehouse and Currys PC World.
The tech company said hackers targeted their systems last year and millions of customers' personal information, names, addresses and email addresses may have been accessed.
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However, no bank details were taken and it had found no evidence that fraud had resulted from the breach.
In an email to customers, chief customer officer Antreas Athanassopoulos said: "On June 13, we began to contact a number of our customers as a precaution after we found that some of our security systems had been accessed in the past using sophisticated malware.
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"We promptly launched an investigation. Since then we have been putting further security measures in place to safeguard customer information, increased our investment in cyber security and added additional controls. In all of this we have been working intensively with leading cyber security experts.
"While there is now evidence that some of this data may have left our systems, these records do not contain payment card or bank account details and we have no confirmed instances of customers falling victim to fraud as a result. We are continuing to keep the relevant authorities updated."
The company apologised to customers and issued the following security advice:-
*If you receive an unsolicited email, letter, text or phone call asking for personal information, never reveal any full passwords, login details or account numbers until you are certain of the identity of the person making the request. Please do not click on any links you do not recognise.
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*If you think you have been a victim of fraud you should report it to Action Fraud, the UK’s national fraud and internet crime reporting centre, on 0300 123 2040
*If you notice any suspicious activity on your bank accounts you should contact your financial provider.