Current laws protecting children from the digital marketing of junk food are "markedly insufficient" and policymakers must take action, health leaders have said.
A World Health Organisation (WHO) report into digital marketing to children outlines a "landscape in which children and food marketers are active, parents have little awareness, and children receive little meaningful protection".
It warned that the food, marketing and digital industries had access to "extremely fine-grained analyses of children's behaviour and exposure to foods high in fat, salt or sugar (HFSS), yet external researchers are excluded from these privately held insights, which increases the power imbalances between industry and public health".
The scenario "presents public health research and policy with challenges on a scale previously not encountered by those whose concern is the health and well-being of children".
Existing rules governing ads for food high in fat, salt or sugar (HFSS) frequently did not apply to digital media, applied only to younger children and not to adolescents, and did not address the "complex" challenges of cross-border regulation, the report said.
Studies had found parents were largely unaware of the "sophisticated, highly engaging techniques" used to advertise HFSS food to children, while digital platforms, marketers and food companies had extensive knowledge of children's activities online, the report warned.
It said the use of tactics such as "zombie" cookies, device fingerprinting and geo-location allowed digital platforms and brands to build extensive, detailed profiles of all those who use the internet, including children from 13 years of age.
The report noted that a study by the British Heart Foundation examining 100 websites for food and drink popular with children found that 80% included products that were not permitted to be advertised to children on television under UK broadcast regulations.
However, the study found that these products were marketed online with cartoons, animations, brand characters, competitions, games, downloadable content such as mobile phone ringtones and links to social networking site product or brand pages.
The report is the first time that researchers and health experts have undertaken a comprehensive analysis of the "concerning situation" in the WHO European Region.
It concluded that "existing regulations are markedly insufficient to address the challenges in this field".
Dr Zsuzsanna Jakab, WHO regional director for Europe, said: "Our governments have given the prevention of childhood obesity the highest political priority.
"Nevertheless, we consistently find that children - our most vulnerable group - are exposed to a countless number of hidden digital marketing techniques promoting foods high in fat, sugar and salt.
"Parents might be unaware of or underestimate the harmful impact of digital marketing, but this report makes clear the effect of such marketing on our children.
"It is the responsibility of policymakers to recognise the new threat resulting from digital marketing of food to children and to act swiftly."
Dr Gauden Galea, director of the division of non-communicable diseases at WHO/Europe, said: "More than 60% of children who are overweight before puberty will be overweight in early adulthood and an estimated 25% of school-aged children in Europe are already overweight or obese.
"This predicts a grim future, as we know that (being) overweight and obesity are a key contributing factor to cardiovascular disease, cancer and diabetes.
"Allowing advertisers and the food industry to market products high in salt, fats and sugars to children through digital platforms with inadequate regulation can have huge consequences in health and economic terms."
Advertising Association director of public affairs Sue Eustace said: "The UK industry has already instigated a far-reaching review of the rules for food ads online, despite our existing regulations being amongst the toughest in the world across all media."
The UK's Committee of Advertising Practice said: "CAP is committed to ensuring food advertising is responsible.
"We will be announcing shortly the outcome of a public consultation on significantly tightening the food rules designed specifically to protect children in non-broadcast media, including digital channels and online."