It noted that many were charging up to 30% more for some products bought in bulk than for the same products purchased in smaller quantities or individually.
The practice was found in almost all of the 30 supermarkets visited last week with an average of 10% of products checked either offering no discount for buying in bulk or proving to be more expensive than buying the same amount individually.
The investigation found that Tesco and Safeway appeared to be among the most likely to be guilty of this practice but smaller numbers of overpriced bulk items were also found at Sainsbury, Waitrose and Asda.
As shoppers would expect it is still cheaper to buy most items in bulk and experts have criticised the stores for exploiting consumers’ expectations that they will always pay less for buying in bulk.
Richard Perks, director of research at the market analysts Mintel told The Sunday Times: “It’s not treating the customer with much respect. Most people assume it’s going to be cheaper and don’t check.”