The names, addresses, dates of birth and bank account details of up to 2.4 million Carphone Warehouse customers may have been accessed in a cyber attack.
Here is a guide on what customers can do to protect themselves.
Look out for suspicious online or account activity
The company cannot be certain of exactly whose data has been accessed, so are advising people to watch out for suspicious activity.
Anyone who notices anything unusual on their account should contact their bank and Action Fraud as soon as possible.
Notify you bank and credit card company
If your bank and credit card company are aware that you could have been affected by the cyber attack, they can monitor your account for any suspicious activity.
Change your online passwords
Those behind the hack may be able to use your personal information to access other online accounts if you use the details for passwords.
Customers have been warned to change any passwords that are linked to their personal details to prevent this happening.
Be wary of any calls or emails asking for personal information
Carphone Warehouse has said it will be getting in touch with customers about the data breach, but people have been warned about fraudsters looking to take advantage of the situation by pretending to be from the firm.
Tony Neate, chief executive of Government-backed web security initiative Get Safe Online, said: "There will always be more cyber criminals looking to exploit the situation and trick you into sharing information a legitimate company would never ask for."
TalkTalk, one of the companies that is provided services by the division affected by the cyber attack, said it would never ask customers for bank details, call people to request they download software onto their computer or send emails asking the recipient to provide their full password.