At sugar levels of 38.4g per 100g the watchdog rated Morrison's Choco Crackles as the 'worst offender' in its top ten list.
“Cereal manufacturers need to wake up to the fact that people want to eat healthily and provide them with the means to do so by reducing sugar and salt levels," said Sue Davies, chief policy adviser, Which?
This latest report, released on Wednesday, will contribute to the ongoing debate on the nutritional content of foods and once again forces into focus ingredients used in formulations.
In the UK, the Food Standards Agency is driving efforts with the food industry to slash the salt content from food products in a bid to reduce the salt intake of the population to 6g per person per day by 2010.
Major industry player Nestle, for example, claims it has reduced levels of the additive in cereals by 30 per cent so far with a quarter of the group's brands containing no added salt.
While industry consortium, the Association of Cereal Food Manufacturers, claims that since 1998 reformulation efforts by its members have brought the total reduction in salt for branded breakfast cereal to 33 per cent.
And commenting to BakeryandSnacks.com on the Which? findings, Morrisons said it has "a concerted programme of development on its own label brands and both of the findings highlighted in the Which report are in fact from old products which have now been completely reformulated".
According to the UK retailer, the new formulation for its own-brand Choco Crackles now has less sugar - 36g per 100g - "bringing it level with comparable products in the category".
Out of the 100 cereals plucked from the UK shelves and investigated by Which? for their levels of fat, saturates, sugar and salt, nearly a third of the cereals - 31 products - contained more than four teaspoons of sugar per recommended serving.
Which? pointed out that Kellogg’s Rice Krispies was the only one out of 28 cereals "specifically marketed to children found not to be high in sugar". The product has 10g sugar per 100g, "but remains high in salt (1.65g per 100g)," said the watchdog.
Joining Morrisons Choco Crackles in the Which? "top ten worst sugar offenders (per 100g)" are: Kellogg’s Coco Pops Moons & Stars (37g); Kellogg’s Frosties (37g); Kellogg’s Ricicles (37g); Sainsbury’s Choco Rice Pops (36g); Tesco Choco Snaps(36g); Nestle Cookie Crisp (35.3g); Nestle Cheerios Honey (35.1g); Kellogg’s Crunchy Nut (35g) and Nestle Nesquik (35g).