The new rules, which come into effect in July 2017, extend an existing ban on television ads for "high fat, salt or sugar (HFSS) food or drink products".
"The new restrictions will lead to a major reduction in the number of ads for HFSS food and drinks seen by children," the Committees of Advertising Practice (CAP) said in a statement.
The body said the ban reflected changing advertising methods and media habits among children.
It cited research saying that children aged between five and 15 in Britain are now spending more time online than watching television.
Children's Food Campaign co-ordinator Malcolm Clark said: "CAP has finally listened to the voices of parents and health professionals, after years of resisting calls for stronger measures".
But Clark and other campaigners pointed out that the ban only applies to media where children "make up over 25 percent of the audience", opening up a potential loophole for junk food marketing.
Jenny Rosborough, campaign manager at Action on Sugar, called for restrictions on programmes such as talent shows which are popular with children but exempt because they are not children's programming.
"We need to see bans on advertising go further," she said.
Food And Drink Federation head Ian Wright said the change was "a major shift" that was "fully supported" by the industry.
Britain is also planning a sugar tax on soft drinks to reduce rising levels of obesity, despite opposition from businesses in the sector.
Britain has some of the worst obesity rates in Europe.
The latest official figures for England from 2014 show that 31.2 percent of children aged between two and 15 were either overweight or obese.
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