Government plans for healthy eating could be blocked by private finance initiative deals in hundreds of schools, which forced them to install junk-food vending machines.
Some of the 450 PFI schools may find it impossible to implement Labour's manifesto for children, launched on Monday, which pledged to tackle public concern about unhealthy food.
Margaret Hodge, the children's minister, said before the launch that schools would be helped to improve meals and already had the power to remove vending machines. "More and more headteachers now are taking out these vending machines that were put in 10 or 15 years ago," she said.
Although PFI companies refused to discuss contracts, governors say that many schools have been forced to install machines selling crisps and sweets in their new buildings. Profits go to the contractors.
Gillian Winders, from the National Governors' Council, said: "People sometimes don't realise that they have signed away their control of food policy. You can get stuck with a contract that you cannot change."
At Harry Carlton comprehensive in Leicestershire, headteacher Graham Legg had to fight to introduce healthy options in just one of their four vending machines.
The manifesto, Children Forward Not Back, promised to halt the rise in child obesity by 2010.
School kitchens, many of which are only suitable for reheating processed food, will be rebuilt, although critics pointed out that much of the funding had already been announced through the Government's Building Schools for the Future programme.
New vocational qualifications will be created for school caterers, and Labour is threatening to ban junk-food advertising aimed at children unless self-regulation works.
A school meals trust would also be set up to help schools provide nutritious diets, and the Office for Standards in Education will be asked to report on canteen food.
"I don't want to see cheap burgers and sausages being put on children's plates. I want to be absolutely clear about that," Ruth Kelly, the Education Secretary, said.
But an Ofsted spokeswoman said that inspectors would not refer to nutritional standards because they were not experts. They would adopt a "common-sense" approach.
The Government denied that it had been shamed into action by TV chef Jamie Oliver's campaign. His petition had 180,000 signatures as The TES went to press.
"It isn't Jamie Oliver who's started the Government on this route," Ms Hodge told BBC Radio 4's Today programme. "We have been working for quite some time. We are the ones who introduced fresh fruit for all children in infant and nursery schools."
The manifesto also promised to protect children from the dangers of the internet by installing parental controls on all the laptops leased to children under the Government's national scheme.
And where Ofsted rates behaviour at a school as unsatisfactory, local authorities will be obliged to provide immediate specialist support.
Schools with behaviour problems will also be reinspected within a year.
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