One in 10 children was obese at the start primary school in England last year but one in five was obese by the end, according to the Health and Social Care Information Centre.
Although figures for Reception children have fallen slightly, the figures for obesity in Year 6 are on the rise.
Children living in the most deprived areas were twice as likely to be obese as children in affluent areas.
Campaigners said the figures should act as a wake-up call.