The compulsory addition of folic acid to all flour in Britain to reduce the incidence of spina bifida could carry unknown dangers for some sectors of the population, child health experts warn today.

An article in the British Medical Journal says the Department of Health should exercise caution before embarking on a national policy that would see folic acid added to flour. A Department of Health committee called for the national policy last year, noting that folic acid supplements could reduce the number of babies born with spina bifida by 41 per cent.

Women are advised in the weeks before and after they conceive to take folic acid supplements to reduce their chances of having a baby with spina bifida. But Professor Brian Wharton of the Institute of Child Health at University College London, and Professor Ian Booth of the Institute of Child Health in Birmingham say there is "no trial evidence of the efficacy or safety" of such an intervention.

The authors accept that "universal fortification" would result in a further fall in the number of babies born with spina bifida. But they are calling for a controlled clinical trial to measure the impact of the supplements on neural tube defects in pregnancy and on less easily defined conditions such as degeneration of the spinal cord and vascular disease in older people.

"Mandatory and universal fortification does not, at present, need the same trial evidence as for a drug," they write. "Yet a drug is not given in imprecise doses to all members of the population without choice or indication.

"Is it acceptable to increase the folic acid intake of 50 million people to prevent a third to two-thirds of these affected pregnancies before there is firm evidence of efficacy and safety in people who are not pregnant?"

Vegetables are the highest natural source of folic acid, but some manufacturers of cereals and breads add supplements. Women can also obtain prescriptions of folic acid.

In the 1980s, there was a 52 per cent fall in the notification of spina bifida. But the United States has reported only a 19 per cent reduction in the prevalence of neural tube defects since national fortification was introduced, which was, "less than half that seen in England and Wales in the 1980s without a fortification programme", the authors say. Folic acid supplements might mask signs of pernicious anaemia in older people, which could then lead to undetected degeneration of the spinal cord, they warn.

The Department of Health concluded last year that adding 240 micrograms of folic acid to every 100 grams of flour could cut the numbers of babies born with spina bifida by 41 per cent. But it said it was still considering, with the Food Standards Agency, whether to advocate compulsory fortification.

Part of the delay was the "need for additional work, particularly on the potential adverse effects of high folic acid intakes". That work was being analysed. Once it was complete health ministers would be able to consider the options.