According to Which? the TV adverts are socially irresponsible, presenting the chocolate straw biscuit as a way of enticing children to drink more milk when in reality the product is unhealthy.
The advert's strapline asks: "How far would you go to get milk into your kids? Well here's an easier way…New Coco Pops Straws make milk more fun."
But the Food Standards Agency (FSA) guideline states 10g of sugar per 100g is 'a lot'. The straws contain 34g per 100g says Which?, over three times the recommended level. And although the suggested serving size on the advert is three straws, this would still provide 10.5g of sugar.
Which? chief policy adviser Sue Davies said: "This advert sends a confusing message about what is healthy and what is not to both children and parents. It is yet another example of the irresponsible and underhand marketing techniques used to push unhealthy food to children."
The move comes as the watchdog campaigns to end the practice of marketing unhealthy food to children, and seems to be using the renowned cereal company as an example of reckless behaviour.
Davies said: "Kellogg's claims that a 31g serving provides at least 17 per cent of the recommended daily allowance of vitamins and minerals. However there is no justification for encouraging children, through advertising, to consume a high sugar product at breakfast when the same vitamin content can be provided by other, healthier alternatives."
And in the US the cereal giant, together with media conglomerate Viacom, will face a lawsuit because of it's marketing of junk food to children.
The foods marketed by these two companies are "directly harming kids' health," claim two parents together with American pressure groups Center for Science in the Public Interest (CSPI) and the Campaign for Commercial-Free Childhood, who are filing the lawsuit.
And a Which? study published in the UK this January revealed marketing devices used to persuade children to demand high-fat high-sugar junk foods are undermining their parents' efforts to curb fat, sugar and salt consumption.
From ambiguous advertising messages to celebrity endorsements, food manufacturers are taking advantage of the government's discretionary marketing code of conduct, ahead of possible legislation that may be introduced in 2007.
Which? has written to Health Secretary Patricia Hewitt MP, calling on her to step in and prepare new legislation, as the voluntary code of conduct system continues to fail.
The watchdog has also kick-started a Kids' Food campaign for responsible marketing that is encouraging parents to get involved and pile pressure on the advertisers.
Meanwhile, nearly one third of two to 15 year-olds in England are now classed as overweight or obese.
And International Obesity Task Force (IOFT) figures released last March show the number of overweight European kids is rising by 400,000 a year, while in excess of 200 million adults across the EU may now be overweight or obese.