Typical school dinners are high in foods linked to anti-social behaviour but lack minerals which experts say help children to stay emotionally balanced. A Soil Association study found that a primary pupil eating five school dinners each week consumed 40 per cent more salt, 28 per cent more saturated fat and 20 per cent more sugar than recommended levels.

By contrast they would receive only 80 per cent of the iron they need and just 70 per cent of the recommended amount of zinc.

Peter Melchett, Soil Association policy director, said such school dinners, which meet official nutrition guidelines, were putting the future physical health of young people at risk.

He called on the Government to take immediate action to tighten the rules in primaries. Ministers are reviewing the standards of secondary school dinners.

There is also a growing concern about the effect of children’s diet on their mental health. Dr Neil Ward from Surrey university told a conference this week that too much sugar and chocolate and a lack of zinc and iron were linked to anti-social behaviour.

Improvements in children’s diets could reduce the need for drugs such as ritalin, which is prescribed to children with Attention Deficit and Hyperactivity disorder, he said.

International studies in juvenile offenders’ institutions showed that changes to young people’s diets could reduce anti-social behaviour by up to 61 per cent without side effects, he said.

In one institution studied by Dr Ward, offenders’ consumption of sugar, chocolate, biscuits, soft drinks, and other foods which are believed to trigger bad behaviour was reduced and inmates were given zinc and iron supplements.

Within 10 weeks there was a sharp reduction in anti-social incidents among those taking part.

Dr Ward said: “The governor said he could not believe that we had taken a young man who was inside for killing both his parents and who was constantly aggressive, and made him human.”

He criticised doctors for being too quick to prescribe drugs and for ignoring research on mineral levels in the body.

A Department for Education and Skills spokeswoman said that the Government was setting up an expert group to look at ways to improve children’s diets.

“We are also undertaking a specific monitoring and evaluation exercise on the quality of primary school meals,” she said.

“We have set down the minimum standards for school lunches, but they are just that - a minimum.”