A row erupted yesterday after an expert said youngsters brought up as strict vegetarians suffered mental and physical problems that could affect them for the rest of their lives.

Nutritionists in Britain dismissed the findings of the US study as " rubbish", and the report prompted Sir Paul McCartney, whose first wife, Linda, put her name to a range of meat-free food, to telephone the BBC to dismiss the claim.

Lindsay Allen, from the University of California at Davis, found just two spoonfuls of meat a day given to children on a vegetarian diet could produce a dramatic and permanent improvement in their physical and mental development. The study took place in Kenya, where children are fed almost exclusively on staple crops. Their diet lacked many of the micro-nutrients essential for the growth of brain and muscle tissue, Professor Allen said. "It's applicable to the West as well. There have been studies on vegetarian women [in Europe and the US] and their children are very developmentally delayed," she added.

Although some vegetarian parents gave their children food supplements, many vegans, who ate no animal products, reared their children on the same food they ate themselves, she said. She told the American Association for the Advancement of Science in Washington: "There is absolutely no question that it's unethical for parents to bring up their children as strict vegans. Even when they were adolescents these children who were fed as vegans when they were young still had delayed development or permanently impaired development,"

Professor Tom Sanders, research director of nutrition and dietetics at King's College London, criticised her for extrapolating from a group in a developing country that had a relatively deprived diet. "Taking people who have limited food choices and adding animal products will provide elements missing from their restricted diets. But where you have a good choice in developed countries, you can select a balanced vegan diet even for children," he said.

Professor Sanders made a study of vegan nutrition which followed children from conception to the age of 26, to show that the development of vegans was normal. "Their diet in developed countries contains plenty of wheat, soy, pulse and salads, and provided they avoid Vitamin B12 deficiency by eating fortified foods or supplements, they are not at any disadvantage," he said. He admitted that a vegan diet for children under the age of five might pose a risk of malnutrition if there was too much reliance on vegetables.

Sir Paul, a strict vegetarian for 20 years, said he had raised his children as non-meat eaters with no ill-effects.

"It has been a good thing for me and my children, who are no shorter than other children," he said. Britain's 500,000 vegans and vegetarians had half the mortality rate of the general population, he added.

Stephen Walsh, of the International Vegetarian Union, said that "to conclude from this particular plant diet that all plant diets are poor, and that the only way to correct the problem is through animal products, is frankly ludicrous".

The study in Kenya involved 544 children with a typical age of seven. Some were fed an extra two ounces of meat a day, while others were given a cup of milk. After two years children fed meat had muscles up to 80 per cent bigger than those with an unsupplemented diet and also showed the biggest improvement in intelligence, activity and leadership skills, Dr Allen reported.