The retail giant’s chief executive Stuart Machin said the data had been accessed due to the “sophisticated nature of the incident” but stressed that this does not include payment or card details, or account passwords.
In a social media post, Mr Machin said there is “no need for customers to take any action”.
“To give customers extra peace of mind, they will be prompted to reset their password the next time they visit or log on to their M&S account and we have shared information on how to stay safe online,” he said.
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