The Sunday Times published an article implying Tesco and Asda were charging unfair mark-ups on Fairtrade bananas and also claimed Sainsbury was overcharging for a range of Fairtrade food products.
Consultant John McCabe took the difference between ordinary bananas and Fairtrade bananas and removed the amount paid to farmers in developing countries to calculate the amount he said supermarkets were pocketing.
In a statement, Sainsbury said: "Sainsbury does not take in an excessive margin on Fairtrade products. Specifically regarding bananas, in addition to higher cost prices, there are other costs associated with distribution, handling and packaging which were not taken into account."
Asda said: "We make less than the average for the fruit and vegetables category on Fairtrade products."
The Fairtrade Foundation said that any comparison of Fairtrade and non-Fairtrade products should compare like with like. The article had compared bagged Fairtrade bananas with loose ordinary bananas.
Deputy director Ian Bretman said: "We don't think supermarkets are acting in concert to keep Fairtrade prices artificially high. We have seen Fairtrade prices come down coffees and teas are now priced very competitively."