Twelve food companies - Coca-Cola, Kraft, Cadbury Schweppes, Kellogg, Burger King, KFC, McDonald's, Haribo, Nestlé, Masterfoods and PepsiCo - were using as many as 20 different marketing 'tricks' to sell unhealthy products, the report claimed.
Promotions included educational worksheets on chocolate, competition prizes that appealed to children and promotional packs to coincide with the release of animated films.
The findings comes a week after Ofcom banned the advertising of food and drink high in fat, sugar and salt in all programmes aimed at children under the age of 16.
“How can parents be expected to give their children a healthy, balanced diet when these sophisticated, underhand techniques are targeting their children, often behind their backs?” said Nick Stace, campaigns and communications director for Which?
A spokesman for Kellogg said: “We are very surprised with the content of this report since the food and drink industry has, and is, voluntarily changing all their marketing practices.”
The report was criticised by the Food and Drink Federation: “It's disappointing that Which? has decided to generate cheap headlines which don't really help to take the debate forward,” said Julian Hunt, the FDF's director of communications.