Jane Wardle of University college, London said tough new rules are needed to halt the increase in childhood obesity and ensure new nutritional guidelines for school meals have an impact on children's health.
Professor Wardle said pupils at risk of obesity would stop eating school meals if given the choice of healthy dinners from the canteen or their favourite junk food in a packed lunch.
She said her research showed 80 per cent of parents of obese children thought they were a "normal weight" and that they were in a state of denial about their children's weight.
Professor Wardle admited it would be difficult to police pupils' eating habits but a first step would be for schools to advise parents on appropriate food.
Professor Wardle, who criticised the National Association of Head Teachers for accepting sponsorship for its annual conference from the fast-food chain McDonald's, has called for a ban on advertising junk food to children.
She says the Government should consider placing cigarette-style warning labels on unhealthy foods. "A lot of the media have been against anything that can be labelled as the nanny state," she said. "My own opinion is we need some nanny state controls."
She believes such measures are needed as children are growing up in an environment which is far more conducive to obesity than that of 30 years ago. The cost of food has declined by 10 to 20 per cent, while improved transport and technological aids mean people do far less exercise.
Professor Wardle said the Government should consider legal restrictions on the size of portions served by fast-food restaurants and increasing the cost to the public of making short journeys by car or public transport.