Any dish containing soya and maize products and tomato paste will have to be checked by school meal providers to ensure they comply with the ban most local authorities have placed on so-called Frankenstein foods.

Up to 60 per cent of processed food could contain genetically-modified soya. Margarines, spreadable fats and salad dressings are also suspect.

Last week, the Local Government Association recommended a five-year ban on GM foods in schools, old people’s homes and town halls. John Ryan, chair of the association’s public protection committee, said: “As major buyers of food, councils should be very cautious on behalf of the public, many of whom are vulnerable, such as children and the elderly.”

One firm, Chartwells, provides school meals in six local authorities. Mike Sparrow, its managing director, said: “Until proper scientific and political debate has resolved the issues of consumer health and safety, Chartwells will not knowingly use GM foods.

“We are requesting continued, written confirmation from our suppliers that their products are free of genetically-modified material. We are considering implementing our own testing programme.”

Gardner Merchant, the largest private provider of meals to state and independent schools, does not knowingly provide foods containing GM products.

But Bill Wadsworth, technical director of Iceland Frozen Foods, said councils should challenge their suppliers. “If councils want to be certain they are not serving these foods to children, they must seek guarantees from their suppliers. We have a set of non-GM suppliers and can advise any council where they can go,” he said.

Although labels should now indicate GM products, some long-life foods such as soups could pre-date the regulations.