Junk food companies use "manipulative tactics" to hook children while they play online, offering free gifts, games and downloads, according to a report.
Food giants including Kellogg's, Rowntree and Cadbury set up websites aimed at children and use Facebook and Twitter to market products to young people, researchers said.
The report, from the British Heart Foundation (BHF) and the Children's Food Campaign (CFC), accuses firms of enticing children to eat food loaded with fat, salt and sugar, and calls for tighter regulation.
Advertising of junk food is banned during children's TV programmes but the report said no strict regulations apply online. It found websites from major brands filled with cartoon characters, videos, competitions, games and apps that appeal to children and the promise of free toys or prizes.
The report said companies employ techniques which many children find difficult to identify as advertising. Few require youngsters to submit their age when entering the website, while companies can also repeatedly contact children directly via email.
The report said: "Companies are exploiting gaps in the regulations to target children online with promotions for products that cannot be advertised on children's television. As a result, children continue to be swamped with commercial messages with one purpose: to persuade them to consume unhealthy products."
More than 75% of the websites studied had high fat, sugar or salt products that were linked to similar pages on social networking sites such as Facebook and Twitter.
Facebook pages allow young people to "like" a product or brand, which means a youngster's Facebook friends are notified of their interactions with that brand, the report said. It argued that the online code regarding advertising is too "vague".
This code says "marketing communications should not condone or encourage poor nutritional habits or an unhealthy lifestyle in children" but the study argued that what "constitutes condoning and encouraging or poor habits is left open to interpretation".
Food and Drink Federation director of communications Terry Jones said: "It is disappointing that the report authors have been highly selective over the information presented in order to make yet another of their seasonal attacks on the food industry. "They have highlighted aspects of our members' online marketing that support their agenda but consciously ignored the many other positive aspects that demonstrate the industry's responsible approach."