The group says that more than 90% of food products advertised during breaks in children's programmes and entertainment shows are full of fat, salt and sugar. The worst offenders, according to Sustain, include McDonald's and Cadbury's.
The group monitored 40 hours of television and 272 commercials shown on ITV on weekday afternoons and Saturday mornings, as well as the ad breaks around Channel 4's Big Breakfast.
Most of the ads in these breaks were for food products, and between 95% and 99% of these contained high levels of fat, sugar and salt. Chocolate, sweets, cakes and biscuits accounted for 48% of food advertisements on Children's ITV and 54% during the Big Breakfast.
The 10 most advertised brands in the study were Viscount Mint Bars, McDonald's, Cadbury's Yowie, Kinder Surprise, Jammie Dodger biscuits, Penguin biscuits, Muller Yogz, Knorr Micronoodles, Haribo sweets and Chicken Tonight sauce.
Sustain accused advertisers of using popular children's characters to target children, and called on the government and advertising watchdogs to take action.