How children eat and play at home is more important than school sports lessons for losing weight, new research suggests. The Government wants all children to do two hours of PE a week, as part of an attempt to tackle the growing obesity crisis.
But a new review suggests that while such lessons are good for overall fitness levels they does nothing to help children to lose weight.
By contrast, teaching entire families to change their behaviour by eating more healthily and taking more exercise does help children to become slimmer.
Ministers are concerned that rising levels of obesity could limit the lifespans of many of today's children.
Last year Alan Johnson, the Health Secretary, warned that youngsters who grew up to be obese faced dying 11 years younger than their slimmer classmates.
Official figures show that one in three children aged 10 are overweight or obese.
A review of 26 studies of different PE programs across Europe, Australia, South America and North America, found that they had little effect on children's weight, or the amount of sport that they played outside of school.
However, the classes did help to lower their cholesterol levels and increased their fitness and lung capacity.
Researchers warned that the reason games lessons were ineffective might be because children disliked them.
"Physical activity classes may be too closely associated with school work, so for some students this makes them feel like they are being made to do more work. Perhaps the key is to promote physical activity by getting children and adolescents to 'play' in ways that promote better fitness levels, while at the same time represent fun and adventurous activities," said Maureen Dobbins, from the School of Nursing at McMaster University in Ontario, Canada, who led the study.
In a separate report, researchers found that encouraging entire families to change their behaviour could help children to lose weight.
A review of 64 studies involving 5,230 people found that children lost up to seven pounds through changing their home diet and activities over 12 months.
"It is now clear that family-based, lifestyle interventions that include a behavioural program aimed at changing diet and physical activity provide significant and clinically meaningful decreases in overweight and obesity in both children and adolescents compared with standard care or self help regimes," said Hiltje Oude Luttikhuis, from the Beatrix Children's Hospital in Groningen, Holland, who led the study.
Both reviews were published by the Cochrane Library.