Their damning report calls for the Government's target for highquality PE and school sport to be increased to three hours in an effort to reverse the "relentless squeeze" caused by pressure from other parts of the curriculum.
MPs on the health select committee warned that both schools and the Government need to do much more to halt the spread of an obesity "epidemic" which if left unchecked could result in this generation being the first to die before their parents.
They called for vending machines selling chocolate and fizzy drinks to be removed from schools and for the Government to reconsider its "unacceptable" endorsement of schemes providing books and sports equipment in return for vouchers from crisp packets or chocolate bars.
The report is the result of the committee's most comprehensive inquiry and will add to growing concerns at the spread of childhood obesity which has more than doubled in the past 10 years.
Children of all ages should study a revamped food technology curriculum which would teach cooking and nutrition, MPs say.
Both food technology and pupils' physical activity should be rigorously inspected by the Office for Standards in Education, they added.
The report condemns advertising which harnesses the "pester power" of children to persuade their parents to buy unhealthy products and calls for government guidance to restrict marketing of such food in schools.
This would include a ban on sponsorship by companies associated with unhealthy food, the removal of unhealthy snacks from vending machines and the promotion of healthy lunchtime options.
Ministers should ensure all children have a healthy school lunch, it says.
David Hinchliffe, committee chairman, said: "It is staggering to realise that on present trends half of all children in England could be obese by 2020. Schools will have to encourage activity and actively monitor the health of their children."
A Department for Education and Skills spokeswoman said: "A healthy body is a healthy mind. We are working to ensure that healthy living becomes a fundamental part of school life."