Experts want iodine to be added to salt after research showed that a lack of the mineral in pregnant women stops half of all children reaching their full academic potential.

Iodine is an essential nutrient for foetal brain development, but studies across the UK have shown that high numbers of expectant mothers suffer from a deficiency of it.

The figures in the six studies range from 40 per cent in Scotland and 47 per cent in the North East of England to 73 per cent in the South West. At 15, the children of mothers who lacked iodine during their pregnancy perform up to 5.6 per cent worse in language tests than their classmates, researchers have found.

A group of leading European nutrition experts warned last month that up to 50 per cent of all newborns do not reach their full cognitive potential because of an iodine deficiency.