Girls as young as seven or eight have been treated at Great Ormond Street hospital for eating disorders, a senior consultant has confirmed.
The hospital has a unit for dealing with children with eating disorders
Dr Jon Goldin said cases of anorexia and related conditions were "very rare" in such young children, but had been seen over 20 years at the hospital.
But he told the BBC News website it would be simplistic to blame all eating disorders on thin catwalk models.
There have been calls for a ban on "too thin" models at London Fashion Week.
Great Ormond Street, the famous children's hospital in central London, has a special unit for treating young children with eating disorders.
Dr Goldin, a consultant child psychiatrist there, told the BBC News website there were many contributing factors to eating disorders.
These could include the onset of puberty, a "perfectionist" personality type and genetic reasons. In a minority of cases, children had been abused, he said.
For others, cultural issues and expectations - a belief that to be beautiful you must be thin - might play a part.
"I think it would be simplistic to say there is a clear relationship between seeing a model on the catwalk and the development of eating disorders," he told the BBC News website.
"There are quite a few different reasons why people develop eating disorders and that could be a contributory factor."
A row over skinny models began during London Fashion Week
But he said children were influenced by images in the media, and it was right there was a debate about it.
"If social values celebrate excessive thinness and if you see highly paid models who are perceived as beautiful, then people are going to think maybe that's the way to get fame and success," he said.
Cultural Secretary Tessa Jowell is among those calling for ultra-thin models to be banned from London Fashion Week - after a similar move by Madrid's fashion week.
Her comments were echoed by Lib Dem London Assembly member Dee Doocey on Tuesday.
"With eating disorders on the increase, it is irresponsible to be glorifying unnatural images that put impressionable girls' and women's health at risk," she said.
'Young women need role models that look like real women, not stick insects on a catwalk."
The British Fashion Council, which runs London Fashion Week, has said it will not tell designers how to run their shows.