The television chef Prue Leith has called for pupils to be barred from leaving school at lunchtime to prevent them buying junk food.

Ms Leith, chairwoman of the School Food Trust, the Government's programme charged with improving school meals in England, argued that locking the school gates would ensure children ate healthier meals or packed lunches rather than burgers or chips.

"If you can keep them inside, then you can begin to educate them about eating," she said. "It's a drastic measure but we are facing a drastic situation. We are denying children the real pleasure of eating and cooking good food. She added: "I agree that I am being rather nanny-ish but I think children need some nannying," she added.

She also advised parents to give pocket money to children in one go on a Saturday, rather than in instalments through the week so they would buy a long-lasting item such as a CD or baseball cap rather than snacks or chocolate.

Only 40 per cent of children eat school dinners. The majority opt for packed lunches or street food. Jamie Oliver's high-profile campaign to improve school nutrition during the Channel 4 series in which he exposed notoriously unhealthy Turkey Twizzlers, has not solved the problem. In many cases, hot meals have been replaced by packed lunches which, said Ms Leith, tended to be less healthy because their ingredients had been bought on supermarket shopping trips when parents were swayed by "pester power".

A new drive by the School Food Trust to encourage children to try the healthier meals and raise the number of pupil diners above 50 per cent got under way yesterday. The Million Meals campaign was launched by Ed Balls, the Secretary of State for Children, Schools and Families, at St Augustine's Secondary School in Kilburn, north-west London.