Petrol has jumped significantly as well, with the latest price for unleaded up by 30p compared to a year ago, according to the RAC.
However, while the price of petrol has increased by 23pc in the past year, the price of milk in supermarkets has on average jumped 27pc higher.
A spokesman for the Co-op said: “Our price is lower than other convenience stores and accounts for increased costs, such as more frequent deliveries and higher rents from running smaller shops in city centres.
“In addition, and unlike some retailers, our dairy farmers benefit from being paid a fair price premium for what they produce to take into account the cost of production.”
Sainsbury’s has also previously blamed the discrepancy on the higher costs of running smaller shops.
“Different sizes of stores have very different operational requirements and running costs,” the company said.
“Rents, for example, are often more expensive in our smaller stores due to their locations.
“It can also be more of a challenge to deliver products to our local stores.
“These, and other factors such as staffing, local rates and a focus on convenience products mean there are differences in price between our supermarkets and local stores.”