Nearly one in four children starting primary school are either overweight or obese, according to government figures which also show that by the time they leave aged 11 the proportion has risen to one in three.

The scale of the epidemic is made clear by yesterday's release of the latest results of the National Child Measurement Programme, which showed that 22.9% of four and five-year-olds are either overweight or obese, indicating that there is a serious problem before children even reach school.

By year 6, it has got worse, with 31.6% of children overweight or obese. In both age groups, boys are more likely than girls to be overweight.

The programme was launched in 2005 with the aim of weighing and measuring every child on arrival in the reception year and again in year 6, the last year of primary school. Up to now, 80% of children have been weighed and measured. From the statistics, their body mass index can be calculated and compared with the ideal for their age and gender.

Childhood obesity is more prevalent in London than anywhere else in England and Wales, with 11.3% of four- and five-year-olds and a huge 20.8% of 10- and 11-year-olds having a severe weight problem. The south-east coast has the fewest overweight pupils - 14.9% in the reception class. The south-west has the fewest overweight children in year 6, at 14.9%.

Stuart Barber, head of policy and public affairs for the British Heart Foundation, found the figures troubling. "These distressing figures confirm that a new generation of children are paying the price for decades of inaction. But it's not too late to reverse the trend."

He urged the government to ban all junk food advertising before 9pm. "Young children are particularly susceptible to junk food advertising, which continually urges them to put their hearts and long-term health at risk."

The Liberal Democrat health spokesman, Norman Lamb, said it was a stark wake-up call to see that more than a quarter of children were overweight before they even started secondary school.

"Worryingly, the scale of the problem may be worse than these figures suggest as the fear of being bullied as 'fat' may mean that some of the heaviest children could be in the 20% of children who weren't weighed," he said.

"It's clear that not enough is being done at the most critical age. Action needs to be taken before children even start school, yet information about healthy living is not getting through to new parents as health visitor numbers decline and school nurses are overstretched."

The public health minister, Dawn Primarolo, said it was encouraging that the measurement programme had reached so many children. "Knowing how many children are overweight or obese and spotting trends is key to ensuring that families, communities, public services, industry and government take the right action in tackling obesity," she said.

The government had recently published a £372m obesity strategy. "We're not going to stem the tide overnight but through measures such as providing the right information and support to parents to ensure a healthy diet and regular exercise for children, and increasing participation in sport and healthy eating, we hope to support families to lead healthier lives," she said.

Body mass index

Body mass index (BMI) is a tool that can be used to tell how healthy a person's weight is. This chart plots BMI against a child's age

Working out your child's BMI

1. Weight in kg, example 30kg

2. Divided by height in metres, example 1.3m

3. Divided by height in metres again

Example 30/1.3 = 23.08/1.3 = 17.75