Drawn up by the Committee of Advertising Practice (CAP), the partner organisation of the Advertising Standards Authority (ASA), the rules apply to media targeted at under-16s and reflect restrictions already in place on TV.
The new rules, which were first announced last December, apply to children's non-broadcast media – including print, posters, cinema, online and social media.
And importantly, ads for HFSS products no longer will be allowed to appear around TV-like content online, such as video-sharing platforms, if it is directed at children.
According to CAP/ASA, children aged between five and 15 spend an average of 15 hours online per week, or more time than they spend watching TV, and so the new regulations come in response to changing media habits among youngsters.
"They also respond to wider concerns in society about the public health challenges surrounding childhood obesity and what part the advertising industry can play in helping to change our children's relationship with less healthy foods," the industry regulator said.
Specifically, the new rules state that ads that directly or indirectly promote an HFSS product cannot appear in children's media, while ads for HFSS products cannot appear in other media where children make up more than 25% of the audience.
In addition, if the content targets under-12s, ads for HFSS products will not be allowed to use promotions, licensed characters and celebrities popular with children. However, advertisers may now use those techniques to promote healthier options.
Commenting on the measures, CAP Chairman James Best said: "The tougher new advertising food rules are a significant and positive change designed to help protect the health and wellbeing of children.
"These measures demonstrate the advertising industry's continuing commitment to putting the protection of children at the heart of its work.
"The new rules will alter the nature and balance of food advertising seen by children and play a meaningful part in helping change their relationship with less healthy foods."
The new restrictions were also welcomed by food and beverage brands in the UK. "We fully support and welcome this landmark move in UK advertising," said Ian Wright, Director General of the Food and Drink Federation, in comments to the Financial Times.
"As young people move away from traditional media towards new and social media, we feel it's important that ad rules keep up with this change," he added.
Data sourced from CAP, ASA, Financial Times; additional content by WARC staff