Coco Pops and KFC have been found guilty of breaking rules designed to prevent firms from promoting junk food to children.
The rulings, published today by the advertising watchdog, set important precedents in terms of establishing how food and drink firms should market products that are high in fat, sugar or salt (HFSS).
KFC ran into trouble for placing a telephone box poster promoting its Mars Krushems drinks near the entrance to a primary school.
Such a promotion is not allowed where more than 25 per cent of those who are likely to see it are under 16.
Meanwhile, Kellogg's broke rules by running a TV advert for its Coco Pops Granola, which is a lower sugar and healthier version of the chocolate breakfast cereal, around an episode of the Mr Bean cartoon series.
The Advertising Standards Authority said while the granola is not itself an HFSS food, the commercial centred on the Coco Pops monkey, pictured, associated with the higher sugar original cereal. As a result, the watchdog said the ad 'had the effect of promoting an HFSS product '
The complaints were brought by the Obesity Health Alliance, whose members include the Royal College of GPs and the British Heart Foundation.
Spokesman Caroline Cerny said: 'The ruling on Coco Pops Granola provides an important precedent for junk food marketing.
‘These adverts are designed specifically to appeal to children with fun cartoon characters and catchy jingles ... This is just another example of the ever-evolving tricks brands use to get their products in front of children.’
KFC said the decision to place the poster near a school was a simple mistake and apologised for the error.
Kellogg's said it has taken steps to improve the health of its products. The firm added that changes made to Coco Pops mean the cereal is no longer classified as a HFSS food, so it can now be advertised on children's TV.