The 'Food Fables' report praises Kentucky Fried Chicken and Weetabix for taking a more responsible approach, but claims that many food companies are still not doing enough to curb their marketing of unhealthy food to children.
Which? found that 88% of consumers think that food companies need to be more responsible in the way they market food to children.
It noted a shift among several of the big companies towards the teen market and young adults, for example, by using social networking sites, text messaging competitions and viral promotions.
Which? said that it found a reduction in some company sponsored website content including games and downloads aimed at young children. However, several companies have developed more sophisticated brand sites, often tying-in with promotions and popular sites such as , , and .
The report also criticised companies for continuing to use cartoon characters such as Kung Fu Panda, SpongeBob SquarePants and Scooby-Doo on food packaging to promote predominantly less-healthy food to children.
It said that tie-ins linked to cinema and DVD releases are continuing at Burger King and McDonald's, with free toy promotions in children's meals.
Yesterday, that it will no longer allow SpongeBob SquarePants and its other characters to appear on food packaging and restaurant promotions for foods high in fat, salt and sugar.
Which? wants TV advertising restrictions to be toughened, as family-viewing programmes like 'Dancing on Ice' and 'Coronation Street' are often watched by more children than a programme like 'The Simpsons'. However, these shows are not covered by restrictions because they focus on the proportion of kids to adults watching.
In a two-week period, the consumer watchdog found that 16 of the top 20 programmes showing ads that were watched by the largest numbers of children did not fall within the restrictions.
The report examined 12 leading food companies including Cadbury, which it found uses web TV tactics such as embedding Creme Eggs into the script of popular Bebo soap Kate Modern.
Coca-Cola introduced a Fanta-branded mobile phone game and used the celebrity sell in its marketing, with footballer Wayne Rooney featuring in several ads and promotions for high sugar products.
Which? praised KFC for restricting advertising to children and removing free collectible toys from kids' meals. It also commended Weetabix for its promotions of healthier cereals to children.
Sue Davies, chief policy adviser at Which?, said: "We're not against treats and we're not against marketing, but we are against irresponsible company practices and hollow company commitments.
"You just have to walk around any supermarket to see the wealth of cartoon characters persuading children to pick the less healthy option."