The TV spot, created by Hogarth Worldwide, advertises brand extension Coco Pops Granola, which is below the thresholds for HFSS. It was shown during Mr Bean on 3 January.
But after a complaint from the Obesity Health Alliance, the ASA ruled that the Coco Pops brand was "significantly more prominent" in the ad than the granola product, due to factors like the size of text in the product logo and the presence of brand mascot Coco the Monkey.
Original Coco Pops has now been reformulated to bring it below the threshold for HFSS – but at the time of the ad, it was classed as HFSS, along with two of the other four products in the Coca Pops range.
Under BCAP rules, both HFSS products specifically, and brands strongly associated with HFSS products, cannot be advertised in programmes aimed at or likely to appeal to children under 16.
Kellogg’s disputed the watchdog’s characterisation of the ad. A spokeswoman said: "we ensured throughout the ad that we were only promoting the Coco Pops Granola product, a cereal that can be advertised in children’s airtime".
She added that the decision was "surprising", because Ofcom had ruled earlier this week that the ad was not in breach of the advertising code.
"We have now reduced sugar in Coco Pops Original by 40%, but kept the same great taste," the spokeswoman added. "This means original Coco Pops now also meets the strict nutritional profiling for foods that can be advertised on children’s TV."
KFC ad shown near school was 'bought in error'
KFC were also in the firing line of the Obesity Health Alliance’s efforts to stamp down on violations of HFSS targeting rules.
The fast food chain was told off by the ASA after it bought a poster site on a phone box near an unnamed primary school, and put up an ad for its Mars Krushems drink.
The watchdog acknowledged that the ad was bought in error – after both KFC’s media agency, Blue 449, and the media owner, Primesight, failed to spot that the phone box was within 100m of the school.
CAP rules state that no medium should be used to advertise HFSS products if more than 25% of the audience was under 16.