Bunker said good reviews for a product could improve sales. But, while Which? selected the main manufacturers of compost to trial, it only used one retailer's own-branded compost - B&Q's.
"Why does it feature?" he asked. "B&Q does not manufacture compost, but merely uses a supplier. In my opinion, this is totally unfair." He added that B&Q's suppliers were also represented elsewhere on the list.
Which? representative Rebecca Leach said: "Which? looks at a number of factors when deciding what products to include in its trials.
"These include whether products are widely available and how popular they are. B&Q has stores across the UK and its compost accounts for a large amount of compost sold in the UK each year."