Parents cleaning out their child's lunchbox at the end of the school day could be in for a nasty surprise — a scolding note from teacher alongside the half-eaten sandwiches and empty crisp packets.

The School Food Trust wants teachers to send out warning letters to parents who fail to comply with school healthy-eating policies. And in advice that could be seen as patronising, the government-funded body suggests further that they send congratulatory letters to those who pack healthy lunches for their children.

Schools across the country were ordered to provide healthy lunches and remove vending machines filled with chocolate and fizzy drinks after a campaign led by the television chef Jamie Oliver exposed the poor standard of meals at many schools.

Most schools also ask parents not to give children crisps, biscuits or similar items for lunch, but the guidance from School Food Trust looks to harden the approach to unhealthy lunchboxes.

In guidance sent as an example to head teachers and governors, the trust lists the foods pupils should not take to school: crisps, chocolate bars, chocolate-coated biscuits and sweets. “Cakes and biscuits are allowed but encourage your child to eat these only as part of a balanced meal,” it says.

Nuts, seeds, vegetables and fruit are welcome, as long as they have no added salt, sugar or fat. “Packed lunches should include at least one portion of fruit and one portion of vegetables every day.”

Most parents are happy to comply with a healthy-eating policy, but could be irritated by the way the trust recommends it is assessed. “Parents and pupils who do not adhere to the packed lunch policy will receive a leaflet in the packed lunch informing them of the policy,” the trust says.

“If a child regularly brings a packed lunch that does not conform to the policy, then the school will contact the parents to discuss this.”

A spokeswoman for the National Confederation of Parent Teacher Associations said: “It's no way to communicate with parents and doesn't put healthy food in a positive light. It may make parents feel underrated, dismissed and dictated to.

“As a parent, I'll sometimes have a biscuit with a cup of tea - everything in moderation is what's needed for adults and children. It makes you wonder what the staffroom lunchboxes look like.”

Justine Roberts, the founder of Mumsnet, the online forum for mothers, said that advice on packed lunches was helpful for parents because it assisted them to say “no” to their children. But receiving a letter would “feel a bit like the lunchbox police”, she said. “Just occasionally there is really nothing in the cupboard because you haven't done the shopping in time, and you just bung in anything.”

She said that the policy should, instead, be made clear at the beginning of each term.

Despite winning praise for making school meals healthier, the Government has been criticised for the way it runs them. The campaign group Food for Life Partnership warned Ed Balls, the Schools Secretary, that the school meal service was “at risk of collapse”.

In a letter it sent to the minister yesterday, it said: “The majority of school meal providers are now running at a deficit which they will not be able to sustain.

“If urgent action is not taken, the Government risks losing this key opportunity to fight obesity and climate change by changing young people's eating habits.

“Unless policymakers start viewing school meals as an education service, not a commercial one, they will end up serving no one.”

The Local Authority Caterers Association backed the partnership. Sandra Russell, chairwoman of the association, said: “School meal providers nationally are encountering financial challenges.”

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