Discounter 99p Stores says it is selling 80,000 packs of potatoes and onions a month through its 46 stores.
It began retailing 5kg packs of large, washed potatoes and eight-packs of onions last autumn, and is now selling 10,000 of each every week.
Business development director Hussein Lalani claimed that the potatoes, at 99p, were the cheapest in Britain.
But he added that 99p Stores was still making a margin of 40% on the produce it sold.
And he insisted: “The produce that we get is of really high quality.”
99p Stores also sells 20,000 18-packs of Spanish eggs every week (also for 99p). Lalani said: “Our egg sales are amazing. We sell out every Saturday morning. Because they’re so cheap we find that they’re popular with restaurants.”
99p Stores recently announced plans to invest £8m in opening 45 new stores in the next 20 months, and to move into Scotland and Wales for the first time.