The government faces fresh criticism this week over Britain's soaring child obesity rates, in a report into the role of schools published by its own education watchdog.

The chief inspector of schools in England, David Bell, will tomorrow issue his first study into the way that healthy eating and the importance of exercise are taught, and will conclude that overall such teaching is patchy and often unsatisfactory.

As head of the Office for Standards in Education (Ofsted) Mr Bell has been concerned about how schools appear to be sending out mixed messages.

According to Department of Health statistics, a sixth of children under 15 in England are obese, and a third are obese or overweight. Amid mounting worries about poor nutritional standards in school meals, Ofsted's report Starting Early: Food and Nutrition Education of Young People, based on inspectors' visits to schools in England, also examines what schools are offering to their pupils through meals.

It will single out some for good practice, but overall it will recommend that schools can improve standards of meals and the way they teach pupils about health and eating.

Mr Bell's report will embarrass the Department for Education and skills, coming just days after the publication of its five-year education plan last Thursday. The strategy said its long-term aim was for primary schools to contribute to reversing increases in obesity.

It said: "Every school should be a healthy school, giving good teaching and advice about nutrition and exercise backed up by its school lunches, by its PE and school sport and ... playground activities. Through this work, we will tackle levels of obesity in children, aiming to halt the growth in obesity among under-11s by 2010."

But the DfES will seek to de flect criticism on the same day, when the schools minister, Stephen Twigg, brings forward the publication of its Healthy Living blueprint for schools which sets out exactly what schools should be doing to teach and promote healthy eating.