Children should be locked inside school grounds to stop them buying unhealthy food from shops and takeaways, a minister said yesterday.

The proposal comes amid new evidence that the Jamie Oliver-inspired drive to make school kitchens offer more nutritious meals is being shunned by pupils in favour of junk food.

The number of secondary school children eating school meals has plunged by 400,000 to barely a third.

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Mothers from South Yorkshire sold junk food to children through the fence of Rawmarsh Comprehensive School in rebellion after shop 'junk food' trips were banned

Children's minister Kevin Brennan said secondary pupils should be barred from leaving the premises during breaks after research found they were spending their money on snacks with high levels of salt, sugar or fat.

Backing a plea by the renowned cook Prue Leith, who chairs the School Food Trust, Mr Brennan said: 'Some schools have a stay-on-site policy for 11 to 16-year-olds but let the sixth form go off- site. I'm very strongly supportive of that approach.

'I would like to see more schools operating some sort of stay-on-site policy because its advantages are shown not just in improved uptake of school meals, but also improved behaviour and community relationships.'

Yesterday's research underlined quite how unhealthy the snacks being bought by children during breaks in the school day are. Some of those who buy their own food during breaks are consuming their entire daily allowance of fat and sugar in one sitting.

A team from London Metropolitan University studied pupils at two large comprehensives - one in a deprived urban setting and another in a well-off suburban area.

The inner-city school allowed all pupils to leave the premises at lunchtime, while the suburban school allowed only sixth-formers the same privilege.

Children's Minister Kevin Brennan has called for secondary school children to be locked inside school grounds during breaks to stop them buying unhealthy food

In the school where pupils were allowed out, just 15 per used their canteen. Even in the school which kept them inside the grounds, less than half (44 per cent) bought food from the canteen, usually sandwiches or wraps, with many buying food on the way to school.

Virtually all the children who were allowed out bought food from local shops, mainly fizzy drinks, chocolate, sweets, crisps, cakes, biscuits and chips.

The researchers found it was not the healthy menus in school canteens that were deterring the pupils so much as long queues, poor facilities and high prices.

They said schools considering keeping children on the premises ought to address these issues first, a finding backed by Oliver last night. 'If you look at what's going on in schools where the catering staff have got the right support and where a "dining culture" is developing, that's where it's working,' he said.

'But there's a big divide between these schools and the many where there are still problems.'

The pitfalls of 'lock-ins' were illustrated at the height of the Jamie Oliver campaign in 2006 when two mothers of children at Rawmarsh Comprehensive in Rotherham started their own takeaway delivery service in response to curbs on pupils' trips to local shops.

John Dunford, general secretary of the Association of School and College Leaders, said: 'Much as schools would like to keep children on site at lunchtime, the number of exits in some - as many as 20 - make this almost impossible.'