Scientists are warning of a potentially “serious risk to human health” after the discovery that traditional varieties of major American food crops are widely contaminated by DNA sequences from GM crops.

In trials, crops have been genetically engineered to manufacture proteins for healing wounds and treating conditions such as cystic fibrosis, cirrhosis of the liver and anemia; antibodies to fight cancer and vaccines against rabies, cholera and foot-and-mouth disease. Conventional drug manufacture is subject to stringent controls to prevent them entering the food chain or contaminating the natural environment. But there are currently no such controls to prevent the spread of DNA sequences from pharm crops.

In tests, up to 1 per cent of individual seeds and more than half of the batches of seeds contained one or more of the GM sequences.