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 today's 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.'
Websites criticised in the report include those from Kellogg's for its Krave cereal, Cheestrings, Nesquik, Sugar Puffs, Capri-Sun, Rowntree, Chupa Chups and Cadbury Buttons.
More than 75 per cent 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'.
Researchers behind the report signed up to the Sugar Puffs website and received emails every week over a three-month period as well as the potential to send branded e-cards to friends.
Users were asked to seek parental permission if they were under 16, but children can simply tick the parental consent 'verify' box.
Cheestrings, which the report said contain more salt than a packet of ready salted crisps, are encouraged to read about '101 things to do before you're 11.5'.
Cadbury Buttons, which the report said contain twice as much saturated fat in one pack (6.2g) as a typical hamburger (3g), features an animated character which can be customised to match a child's name and features.
The website asks 'supervising grown-ups' to enter their year of birth although experts said this could easily be bypassed by a child entering a false date.
The Kellogg's Krave website features the Krave Krusader, who is described as 'fantastically fast-paced fun'.
Researchers found there was no age verification required to enter the website, but to sign up for the newsletter users must state they are at least 16.
To visit their Facebook page, users are told they must be 17 but the researchers said this could easily be bypassed.
Mubeen Bhutta, policy manager at the BHF, said: 'Like wolves in sheep's clothing, junk food manufacturers are preying on children and targeting them with fun and games they know will hold their attention.
'Regulation protects our children from these cynical marketing tactics while they're watching their favourite children's TV programmes but there is no protection when they're online.
'The marketeers must be rubbing their hands with glee because this loophole gives them carte blanche to reach eight in 10 children behind their parents' backs.'
Charlie Powell, campaigns director for the CFC, said: 'By its failure to protect children from online junk food marketing, the Government is demonstrating complacency at a time when it should provide robust regulation to help reverse unacceptable levels of obesity in the UK.'
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.
'For example, the sites are clearly branded, there is no attempt to mislead consumers and parental interaction is encouraged.
'Advertising in the UK is well governed and rules have recently been revised to include online material.'
Paul Wheeler, from Kellogg's, said: 'It is absolute rubbish to say Krave's digital marketing is aimed at kids as you need to be 17 to follow Krave on Facebook, play the Krave game and take part in our Krave vote.
'And only people 16 or over can sign up to the Krave newsletter.
'The report's authors would have discovered this if they'd bothered to check their facts with us.'
A spokesman from the Department for Culture Media and Sport said: 'Most online space has been covered by advertising codes for some time, but in 2010 the Committee of Advertising Practice extended online protections further.
'Regulations now cover promotional activity by companies on their own websites, on social networking sites and the use of adver-games and user-generated content.'