The disruption in supply has come about because the firm has had to take some of its food-related systems offline. It is using different processes to improve availability, so it can operate as normally as soon as possible.
In M&S's Marble Arch store in central London, signs on some of the food shelves that were missing items said: "Please bear with us while we fix some technical issues affecting product availability."
Dot, 52, who shops at M&S regularly, said some of the shelves were quite empty.
"I was looking for my favourite biscuits and couldn't find them," she said.
Ken, 76, also said the limited stock was "definitely noticeable", although the staff were "perfectly charming" considering the cyber attack.