The commission, an independent food watchdog, says most of the discounted foods are products such as burgers, chips, crisps, jams, biscuits, cakes, icecream and soft drinks.
Their quality is often "substantially inferior", it says. Supermarkets, despite having launched healthy-eating initiatives, appear to be promoting "anything they think will sell, with a sorry disregard for the nutritional effects on their customers".
The discounts have been introduced by chains such as Sainsbury, Safeway and Tesco in response to the "pile them high and sell them cheap" tactics of stores including Kwik Save, Aldi and Netto, which have begun to threaten the giants' dominance.
Tesco's Value lines were followed by Safeway's Savers and Sainsbury's Essentials. Kwik-Save fought back with its No Frills range. Many discounted products make a loss, but that is judged worthwhile in the fight to regain customers. Tesco estimates its Value line brings in an extra half-million people a week.
Price is crucial, particularly among low-income groups already likely to suffer the worst diets, in deciding purchases, the commission says. But the chains have ignored a "perfect opportunity" to use pricing policies to encourage healthier eating.
Department of Health guidelines on avoiding heart disease and obesity say a third of our diet should be fruit and vegetables, a third bread, potatoes and cereal foods, and no more than 7 per cent should be fatty, sugary foods.
However, fruit and vegetables account for only 10-20 per cent of discounted lines, while fatty, sugary foods make up at least 25 per cent, rising to 34 per cent at Safeway and 38 per cent at Kwik Save, according to a survey by the commission published inLiving Earth and the Food Magazine.
Only two of the four chains surveyed discounted potatoes, carrots and onions. Sainsbury's, while offering the most fruit and vegetables at discount prices, "happily included" sausage rolls, cola drink and granulated sugar among cut-price Essentials.
All discounted white sliced bread, but only two discounted wholemeal and both, Safeway and Tesco, charged 40-50 per cent more for the wholemeal.
The poorer quality of discounted lines is also criticised. Fish fingers were minced fish, burgers were likely to include poor-quality or mechanically recovered meat, pies and pastries had more pastry and less filling, and ham had extra water, it says.
Cut-price promotions also showed a "notable absence" of comprehensive nutritional labelling: some products gave the "bare minimum" and others none at all.
Dr Tim Lobstein, the report's author, said the stores were "encouraging poor diets among those who have to watch their pennies", people who were already suffering high rates of obesity and diet-related ill-health.
"It contradicts the efforts made by the Government and indeed contradicts the supermarkets' own avowed policies of encouraging better diets."
Safeway said 41 of the 67 foods on its Saver range were products "which any health-conscious shopper would be proud to buy". Sainsbury said its Essentials range was aimed at everyday items and was "totally separate" from its healthy-eating initiative.