Leading food manufacturers are accused of using tactics to push foods high in fat, sugar and salt to children.
Guidelines on food advertising are being tightened
A report by consumer group Which? says companies use "underhand" ploys such as free toys and websites to appeal to young children.
Researchers monitored the marketing practices of 12 food giants over six months and interviewed 50 children.
Food manufacturers rejected the consumer group's claims, saying they adhered to advertising guidelines.
Regulator Ofcom announced last week that junk food ads during TV programmes targeted at under-16s will be banned.
There will also be a total ban on ads during children's programmes and on children's channels, as well as adult programmes watched by a large number of children.
The food industry says these restrictions are "over the top", but health bodies and Which? says they do not go far enough and that all junk food ads before the 9pm watershed should also be banned.
Which? researchers looked at 20 different marketing techniques used by the 12 companies: Burger King, Coca-cola, Cadbury Schweppes, Haribo, Kellogg's, KFC, Kraft, Masterfoods, McDonald's, Nestlé, Pepsico and Weetabix.
The most popular methods used the internet, children's films and the World Cup.
Some products encouraged "viral marketing", when children spread a brand message to each other by emailing e-cards, cartoons or spoof adverts. Others used competitions.
Which? says advertisers are turning their focus to parents too, "making foods high in fat, sugar or salt seem healthy options for their children".
Nick Stace of Which? said: "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?
"Most of their so called responsible marketing policies are simply empty rhetoric."
But the food industry condemned the report.
Julian Hunt of the Food and Drink Federation said: "It's disappointing that rather than work in this spirit of cooperation and partnership, Which? has decided to generate cheap headlines which don't really help to take the debate forward."
He said the industry was looking to work constructively with the government and other organisations on all aspects of marketing to children.
Kellogg's spokesman Chris Wermann 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 in partnership with the government, consumer groups and Which?
"This is further evidence of a sensational report that is selective, ill-informed and unrepresentative, and simply does not give parents the whole picture.
"Not only that, it uses information that is out of date, misleading or plain wrong."
McDonald's said: "We completely reject the notion that any of the methods by which we advertise are 'underhand', or that we market so-called 'junk food' to children.
"McDonald's adheres to all current codes of practice and this will continue as the new regulations announced by Ofcom recently are introduced."
It said it had made changes to its menu which means more items are not classified as unhealthy.
But Jenni MacDougall of Cancer Research UK said: "The techniques used to promote foods high in fat, salt and sugar are very creative, and many of them hold appeal for children. With childhood obesity reaching alarming levels, it is vital that the problem is tackled from all angles."
She added: "We are disappointed that Ofcom has not gone further in its proposals to restrict junk food advertising to children."