The government has admitted for the first time that almost half of all children will be dangerously overweight by 2050 if drastic action is not taken to halt the growth in childhood obesity.

Revealing the true scale of the obesity epidemic, Ed Balls, the Secretary of State for Children, Schools and Families, said the number of children in the high risk category was rising at a worrying rate.

'If that trend continues, by mid-century we'd be getting close to half of all children classified as obese,' he said.

Balls highlighted the fact that 12 years ago, just before the Labour party came to power, fewer than 10 per cent of primary schoolchildren were classified as obese. That has jumped to 16.9 per cent of boys and 16.8 per cent of girls, aged between two and 10.

His concerns were echoed in research revealed by The Observer earlier this year, which suggested adult and childhood obesity would continue to rise relentlessly into the middle of the century. Ministers commissioned the work but refused to comment on its conclusions. Now Balls has spoken up, saying: 'It is clearly important that we act and that is what we are doing.'

When children across England go back to school this week it will be under new rules that ban the sales of all chocolate bars, flavoured biscuits, sweets, crisps and cereal bars. Salt will no longer be provided on tables, ketchup and mayonnaise will be limited and cakes will only be allowed at lunchtime. The regulations add to those already governing the content of school lunches.

Since he has been in his new position, Balls admitted he had come under pressure from food manufacturers to 'ease, delay or soften' the new restrictions. But because of the 'scale of the problem', he refused. The minister also announced £150m for new kitchens in schools that do not have them and new guidance on how to build an attractive space for children to eat in.

Balls also turned to the issue of gang culture and schools, and the horrific murder of 11-year-old Rhys Jones. 'We certainly see schools as playing an important role in this wider effort to tackle gangs and gun crime,' said Balls. He said he would like to see the 'safer school partnership', which links 400 schools up with the local police, rolled out into all areas with gang problems.

When it came to healthy eating, head teachers said the changes would be worthless without parents on board.

John Dunford, general secretary of the Association of School and College Leaders, said: 'Headteachers are keen to support the anti-obesity measures including the provision of healthy food in schools but children do most of their eating outside school and education for parents is top priority.'

The healthy eating agenda has already suffered a number of setbacks including dinner ladies threatening to strike because of the extra work.

Moreover, since Jamie Oliver first launched his school dinner revolution on television in 2005 the number of pupils opting for the meals has slumped. At one school in South Yorkshire, a group of mothers pushed burgers through the school gates because children were so unhappy with the new, healthier food. Most will not have to go as far as that. Balls admitted he was powerless to stop parents putting unhealthy food inside lunchboxes.

Research released tomorrow reveals that children who eat packed lunches are less healthy than those who eat in school, and the gap is widening.

A study carried out for the School Food Trust (SFT), set up by the government, found those eating school meals were more likely to eat vegetables and fruit and less likely to snack on chocolate or crisps.

Sixty-five per cent of lunchboxes in the study contained an unhealthy snack and 31 per cent included a drink that did not meet the food-based standards already in schools. In an attempt to change the trend, the SFT are this week rolling out guidance for headteachers on how to put in place a 'lunch box policy'.

Parents may be surprised by just how far they have to go to match the nutritional standards set in schools. An SFT example of a lunchbox that would be good enough included 'sliced roast beef with a mixed vegetable rice salad' on one day and a 'chicken tortilla wrap with sweet pepper, carrot and tomato slices' on another.

'A lunch at school offers a child more opportunities to try different foods, more choice and is a lot less hassle,' said Judy Hargadon, chief executive of the SFT. She said it was preferable for children to opt for hot meals in school, and that would continue to be the focus for her organisation.

Anita Bean, a nutritionist and author of Healthy Eating For Kids said that parents should revert to methods of the past: 'A common mistake for parents is to give the child too much choice [over food]. The old-fashioned rule of just eating what was put in front of you seems to have waned. The children shouldn't get into the habit of rejecting food, which gives them power and control. Don't let them demand food.'