Organic milk has about a third less iodine than conventionally-produced milk and may impair the development of babies' brain in the womb, a new study says.
Pregnant women who switch to "healthier" organic milk may be putting the brain development of their unborn babies at risk, experts have claimed.
Milk certified as organic contains about a third less iodine than conventionally-produced milk, according to a new study. The same was found to be true for "ultra-high temperature" (UHT)-processed long-life milk.
Since milk is the primary source of iodine in the UK diet, the discovery is said to have potentially serious health implications.
Iodine is known to be important for the healthy brain development of babies, especially in the early stages of pregnancy.
Previous research has shown that mothers-to-be who are iodine deficient during this critical time can give birth to children with reduced IQs.
Lead scientist Professor Ian Givens, from the University of Reading, said: "People are increasingly buying organic and UHT milk for perceived health benefits or convenience. But our research shows that this trend could have serious implications for public health.
"Iodine deficiency ought to be a health problem from the past. But unless this situation is carefully monitored, we risk sleepwalking into a new health crisis in the 21st century.
"Organic and UHT milk is not bad for you, and drinking all types of milk has numerous health benefits. But to get the same amount of iodine as in a pint of conventional pasteurised milk, you would need to drink around an extra half-pint of organic or UHT milk."
Iodine deficiency was once endemic in parts of the UK, with hundreds of thousands of people in the 1930s and 1940s suffering from goitre.
The condition, caused by a lack of iodine, is marked by abnormal swelling of the thyroid gland in the throat.
In some countries iodine is added to bread or salt, but in the UK supplements given to dairy cows led to milk becoming the primary dietary source of the element, said the researchers.
Experts have described the milk-linked virtual elimination of iodine deficiency in the UK by the 1990s as an "accidental public health triumph".
But 20 years ago doctors warned that changing trends in the production or consumption of milk could affect iodine uptake.
Up to 70 per cent of teenage girls across the UK are now iodine deficient, probably as a result of a decline in milk consumption, said the scientists, whose findings are reported in the journal Food Chemistry.