Healthy options offered by burger and pizza chains are still stuffed with salt and fat despite menu changes.
An investigation of the food sold by the "big four" - McDonald's, Burger King, KFC and Pizza Hut - found that 17 of 20 products were high in salt or saturated fat or both. Of those, five out of eight of the salads used as "evidence" of their embrace of healthy eating had "high" salt or fat content.
On average, the fast-food meals sampled by Which? had 274 calories per 100g of food, more than double that of a home-cooked roast dinner. And there were inaccuracies in the nutritional information provided by three of the companies.
McDonald's website claimed that a Big Mac and medium fries had 786 calories but analysis showed it had 900. Burger King's Whopper and regular fries had 19 grams of saturated fat, rather than the 13 grams claimed. Levels of saturated fat in KFC's Zinger crunchy salad were almost treble the company estimate.
Obesity has tripled in England since 1980. A third of children aged two to 15 are overweight or obese. Which? said that although fast food was not solely to blame, the rise in weight had been accompanied by the rise in fast-food sales.
Researchers said the chains frequently targeted children in promotions by giving away toys or goodie bags. Many used children's characters such as Mr Men, Postman Pat, Winnie the Pooh and My Little Pony.
Which?found the popularity of fast-food outlets was related to their advertising budgets. Researchers also analysed nutritional content. Burger King fries were only 86 per cent potato; the 11 other ingredients included partially-hydrogenated vegetable oil, rice flour, dextrose, corn syrup solids and salt. McDonald's chicken grills contained 19 other ingredients. McDonald's and Burger King's cheddar slices included "cheese flavouring", trisodium citrate, diphosphates, polyphosphates and sorbic acid.
Some of the fast-food meals scored astronomical calorific counts. A Big Mac, medium fries and small vanilla milkshake contained 1,169 calories. A diner would need to walk 16 miles to work that off. Which? said: "Nearly all the fast food we tested contained a lot of salt. And salt can lurk where you least expect it. The KFC original chicken salad contains more salt than the KFC chicken fillet burger."
A Pizza Hut margherita pan pizza and garlic bread had 5.4 grams of salt, almost the entire recommended daily allowance of 6 grams.
The companies said their menus now had more variety. Pizza Hut said it gave customers "the choice of healthy or more indulgent food". McDonald's said its customers visited on average just two or three times a month.
Tim Lang, professor of food policy at City University, said: "The food industry knows there is evidence of a health problem which it is part of, but it likes to claim it has nothing to do with it."
Which? wants to ban television advertisements for fast-food aimed at children, saying such "aggressive but sophisticated" marketing was irresponsible.