High levels of a fungal toxin have been found in organic maize products sold in British shops, prompting top scientists to question the safety of this increasingly popular approach to agriculture.
Experts have called for research into the possibility that organic foods may be more prone to contamination by potentially harmful substances produced by moulds.
A mycotoxin called fumonisin was detected in maize meal and flour tested by Food Standards Agency scientists in September. All contaminated products were voluntarily withdrawn from sale.
The case has stoked fears that the spread of farming practices eschewing modern pesticides and fungicides may lead to a rise in mycotoxin poisoning.
Some scientists have even suggested the cereal crop fungus ergot, which killed thousands in medieval Europe, might make a comeback as a result.
Ian Crute, director of Rothamsted Research, the government-funded agricultural research institute, warned: "The lack of control of plant pathogenic fungi - such as ergot - that have the potential to produce toxic metabolites is definitely an 'achilles heel' for organics and is a food scare waiting to happen."
A spokeswoman for the Soil Association, which regulates and campaigns for organic farming, backed the call for research but dismissed the scientists' concerns.
"Comparison of the levels of these compounds and the possible risks associated with them is of great importance, particularly when compared with the possible effects of the cocktails of fumigants and fungicides that may be detectable on non-organic products," she said.
She added: "There is no real reason or evidence that organic farming has higher levels of mycotoxin."
Consumption of fumonisin, which is produced by the mould fusarium, has been linked to liver cancer and immune system damage in laboratory animals.
Fumonisin was found in minute quantities in all 32 maize products screened.
But the eight organic brands - as well as four conventional ones - exceeded limits being considered for the mycotoxin.
A working document produced this month by the European Commission's agricultural contaminants expert committee suggests a maximum level of fumonisin in food of 500mg per kilogram for adults and 100mg per kilogram for infants.
One organic maize meal tested by the FSA contained 20,435 mg per kilogram - 200 times the level thought safe for children.
The FSA stated this was "unlikely to be any immediate risk to health". But a spokeswoman admitted there might be problems "if eaten at high levels over a long period of time".
She noted FSA screening had not found heightened levels of mycotoxins in any other organic foods.
"There is not enough information available at present to say that organic foods are significantly different in terms of their safety and nutritional content to those produced by conventional farming," she added.
The FSA is consulting over a research programme to compare pesticide residue and nutrient levels in organic and conventional foods.
Some scientists felt this should be extended to include mycotoxin levels.
Michael Wilson, chief executive of government-funded Horticulture Research International, said: "We don't have a proper grasp on the problem because no systematic analysis has been undertaken."
He said many factors could influence mycotoxin levels, such as growing-season climate and insect damage to a crop.
He called on the FSA to conduct research into the potential problem, adding: "If any GM product had the levels of toxin (found in the organic maize), it would be the end for GM."
The FSA is midway through a five-year investigation into the environmental factors that prompt the production of fumonisin and other toxins in both conventionally and organically farmed oats, wheat and barley.
Jim Duncan, a senior scientist at the Scottish Crop Research Institute, said: "By not applying normal plant protection measures, such as fungicides, organic food would appear to be more at risk from mycotoxin contamination."
But Ray Coker, professor of food safety at Greenwich University, insisted a full survey was needed.
"I've heard such concerns raised before but the jury is still out on whether, for example, not using fungicide could lead to higher levels of mycotoxins," he said.
Tony Trewavas, professor of applied biochemistry at Edinburgh University, has written to the FSA calling for an investigation into the potential problem.
"No one knows what fumonisin levels are dangerous," he said.
Richard Mithen, head of plant foods for health protection at the Institute of Food Research, said: "I have been concerned that more widespread adoption of organic systems will lead to a resurgence of diseases such as bunt in cereals and, with important implications for human health, a resurgence of fungal diseases that produce toxins for consumers."
Peter Goodenough, principal research fellow at Reading University and editor of the International Journal of Food Science and Technology , said:
"If growers in some climatic regions regularly grow their crops without fungicides, sooner or later ergot poisoning will occur again."
But the Soil Association spokeswoman said anecdotal evidence suggested organic crops were less susceptible to fungi than conventional crops, as they possessed thicker plant-cell walls.
She said fungal infections were best controlled through crop rotation, lower applications of nitrogen and the selection of resistant crops.