Marks and Spencer has started to sell over-sized school uniforms for overweight children as young as four.
Its 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."
Mr Fry said there needed to be a collective effort to curb obesity.
"Parents should not fail in their responsibility - it is they that put food in their children's mouths, send their children out to play.
"But at a government level, they have consistently ducked out of regulating the food industry.
"If you allow the food industry to self regulate - and the government sanctions the fact that they are not going to regulate - then the food industry will just carry on making the food it is making."
A spokesman for Marks and Spencer said: "It is a small online trial running in response to customer demand. Marks and Spencer is the leading schoolwear retailer and we want to make sure our schoolwear range is accessible for children of all shapes and sizes."
More than one in five children in England start their school life overweight or obese, figures from the NHS Information Centre released last December show.
By the end of primary school the rate rises to nearly one in three, the government's child measurement programme found.
The schools data showed more boys than girls were overweight in both reception and year six. Some 24% of boys aged four to five were overweight or obese, while 21.5% of girls were.
In the 10 to 11-year-old age group, 34.5% of boys and 30.7% of girls weighed too much.