The outspoken heir-to-the-throne was at a conference organised by his own Prince's Foundation for the Built Environment and the King's Fund.
The prince said that research by Dr Richard Jackson showed that Brits were getting fatter because of poor urban design: "it might help if the built environment was more attractive and appealing to the pedestrian," he said.
The Prince also backed bikes:
"Research... suggests that walking or cycling for just half an hour a day can have a significant improvement on our state of health," said the Prince of Wales.
If Brits don't get on their bikes, they risk becoming as obese as Americans, said the Prince:
"We are perhaps not far behind our American cousins in the 'supersizing' epidemic."
Prince Charles praised walking and cycling's potential life-saving capabilities.
"But why don't we do it more?" he asked.