Getty ImagesMarks and Spencer (M&S) says it has been dealing with a "cyber incident" affecting some of its services over the last few days.
The UK retailer said its Click and Collect service has been impacted by technical issues along with its ability to collect contactless payments - with many customers taking to social media to complain about delays.
M&S chief executive Stuart Machin apologised to customers in a note on Tuesday.
He said the company had been forced to make temporary, small changes to store operations "to protect you and our business".
"There is no need for you to take any action at this time and if the situation changes, we will let you know," he said.
The notice said the firm was working to resolve some "limited" delays to Click and Collect orders.
It comes after some shoppers complained over the weekend about the issues - which also include being unable to use gift cards or vouchers in stores.
One person called the issues a "total failure for customers" in a post on X.
"A simple message out to customers to save a journey would have worked a treat," they said.
And another said they were unable to pay for clothes using a gift card while shopping at a M&S store in Liverpool.
