A gaggle of schoolchildren at the chip shop is a common scene in Britain's high streets - but it may not be for much longer. The children's minister, Kevin Brennan, has called for secondary pupils under the age of 16 to be locked in school grounds at lunchtime to stop them from stocking up on sweets, fizzy drinks and takeaways.

The proposal comes as damning new research reveals the extent to which children pass through school gates to buy large quantities of food that is high in fat and sugar. Some pupils left school to buy junk food more than 11 times a week.

With soaring numbers of children now dangerously overweight, Brennan said one answer was to keep millions of pupils inside the gates. 'Some schools have a stay-on-site policy for 11- to 16-year-olds but lets 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 healthy school lunches], but also improved behaviour and community relationships.'

But any plans to lock children in were attacked as Orwellian by parents yesterday, while headteachers argued that it was not possible to police pupil movement. 'Much as schools would like to keep children on site at lunch time, the number of exits in some - as many as 20 - make this almost impossible,' said John Dunford, general-secretary of the Association of School and College Leaders.

Andy Hibberd, co-founder of the Parent Organisation, a support group, said the proposals should be opposed and blocked. 'It is very "nanny state-ist", totally unworkable and impractical,' he said. 'It is taking away parental choice and treating older children as babies.'

But others were more supportive. Cathy Court, a director of Netmums, a mothers' social networking organisation, said: 'My own view is that 16-year-olds are still children and should stay in,' she said. 'They need to stay on site not just to stop them buying unhealthy stuff from local shops but also for safety reasons.'

In the first study of its kind, academics from London Metropolitan University mapped the movements of hundreds of pupils over a week. In some cases pupils consumed their daily recommended allowance of fat and sugar in one sitting. The key findings were:

· Supermarkets, newsagents and takeaways were far more popular than the school canteen, with 80 per cent of children using them regularly.

· The top 10 purchases made by pupils included fizzy drinks, chocolate, sweets, chips and fried chicken. Fruit and vegetables came 22nd out of 26 food types.

· Girls' eating habits were less healthy than those of boys.

· Pupils snubbed school lunches because they disliked the queues, lack of seats and high prices.

· Only 6 per cent of pupils ate hot lunches, fewer than half those entitled to free school meals.

· A breakdown of the nutritional values of the most popular purchases revealed that young people were consuming dangerously high levels of fat and sugar.

· A chip shop near a school featured in the study sold 63 children's portions of chips to pupils within half an hour of lessons ending.

It is three years since chef Jamie Oliver first exposed the appalling state of school food through his Channel 4 series Jamie's School Dinners. His campaign was hugely successful, forcing ministers to introduce new nutritional standards and provide £280m to pay for it. But offering healthy food was not the same as persuading pupils to eat it.

Now that experts have shown just how far children will go to seek out fatty and sugary snacks, ministers will have to turn their attention to the fast-food outlets and supermarkets ringing schools.

Last night Oliver welcomed the research. '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. But there's a big divide between these schools and the many where there are still problems,' said Oliver.

The authors of the latest research condemned local councils for contributing as little as three pence towards the cost of each school meal. 'That is totally inadequate,' said Sarah Sinclair who worked with Jack Winkler, professor of nutrition policy.

Food blog: Junk food is pushed at our kid