Tesco milk on sale in Sainsbury's in Cambridgeshire store
A Sainsbury's store offered a competitor's milk for sale before the error was pointed out by customers at the Cambridgeshire branch.
Shoppers in Ely noticed Tesco milk next to the supermarket's own-branded milk.
A Sainsbury's spokesman said: "Our supplier accidentally delivered us some milk from a competitor, a small amount of which made it on to our shelves."
He said it was replaced as soon as the mistake was pointed out to staff.
"We hope our customers in Ely weren't offended by this and we would like to promise them this won't happen again," he added.
A Tesco spokesman said: "It's good to see Sainsbury's has got great taste in milk."