Eight-year-olds whose mothers ate large amounts of the sweet while waiting to give birth did less well in intelligence tests than other children, a study found. They were also more likely to have poor concentration spans, are more likely to misbehave and struggle in school.

A component in liquorice called glycyrrhiza is believed to damage the placenta, allowing stress hormones to cross from mother to baby. High levels of the hormones may affect foetal brain development and have been linked to behavioural disorders in children.

The study, co-led by researchers from the University of Edinburgh, involved testing mental abilities in 321 eight-year-old tots — including vocabulary, memory and spatial awareness. Behaviour was assessed by means of an in-depth questionnaire completed by each mother.

It showed women who ate the equivalent of 100 grams of pure liquorice were at greater risk of having children with reduced intelligence levels and behavioural problems, the scientists reported in the American Journal of Epidemiology.

Professor Katri Raikkonen, from the University of Helsinki in Finland, said: "Expectant mothers should avoid eating excessive amounts of liquorice."