The Prince, who has a fleet of chauffeur-driven cars and has rarely if ever been seen in public on the saddle of a bike, said: “We are perhaps not very far behind our American cousins in the ‘super-sizing epidemic’.”

In his speech at St James’s Palace he gave warning of a worrying sharp rise in childhood obesity. His intervention comes after the British Medical Association said that Britain’s fat youngsters, who account for a third of all obese children in Europe, were at high risk of developing life-threatening conditions, including diabetes and heart disease. One million under-16s were so fat that they were putting their health at risk, the BMA said.

The most recent statistics from the Department of Health indicate that more than 40 per cent of men and 33 per cent of women are overweight and 20 per cent of both are obsese.

The Prince, who credits his trim figure to a diet of one organic meal a day, which he calls the Des O’Connor diet, and long walks on the Scottish moors when he is at Balmoral, said that the rise in obesity was partly caused by the design of modern towns and cities.

“Research suggests that walking or cycling for just half an hour a day can have a significant improvement on our state of health. But why don’t we do it more?” The answer, he said, was that too many towns made it virtually impossible. “It might help if the built environment was more appealing and attractive to pedestrians.”

He urged the Government to embrace the work of Richard Jackson, a former adviser on health to Arnold Schwarzenegger, the Governor of California. The Prince said: “Dr Jackson and his colleagues have pointed to a disturbing link between the built environment, physical inactivity and what he terms a syndemic of diseases, including, perhaps most worryingly, childhood obesity.”

Last year Ruth Kelly, the Education Secretary, and Margaret Hodge, the Children’s Minister, said that junk food advertisements during children’s television programmes would be banned by law if the industry failed to agree a voluntary code. They were contradicted by Tessa Jowell, the Culture Secretary, who said that the scheme would undermine income for ITV.

The topic of how building design affects public wellbeing is one of the Prince’s favourite hobby horses. He set up the Foundation for the Built Environment, which hosted last night’s event, to promote traditional urban design and architecture that put communities at the centre of the process.

In the past, he has called for greater integration of complementary medicine, to be used to tackle obesity, especially in youngsters. In November he backed demands for healthier, more nutritional, school meals.

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