The new Plus schoolwear range includes clothes for pre-school children with waistlines of up to 23ins, a size usually worn by eight-year-olds.
Campaigners said it was simply commercial recognition of the fact obesity was a growing problem among younger pupils.
Marks and Spencer said the trial range followed demand from parents.
The range, which started being sold online last week, caters for ages three to 16, with trouser and skirt sizes going up to a 41-inch.
Tam Fry, of the National Obesity Forum, said: "This is the actual commercial recognition of what we have known for some time - that obesity in pre-schoolers is building up. Now 27% of entrants to primary schools are overweight or obese."
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