Chemicals found in beauty products such as lipstick, nail polish and hairspray cause damage in the womb that can significantly reduce a child's intelligence, research has suggested.

Exposure to the highest levels of two of the phthalate chemicals led to a lowering of IQ scores at age seven by more than six points, scientists found.

The link remained after taking account of known factors that can influence child IQ, such as the mother's intelligence and education, and the quality of the home environment.

Researchers in the US investigated four phthalates and demonstrated an association with two, known as DnBP and DiBP.

Both are found in a wide range of consumer products including dryer sheets, vinyl fabrics, lipstick, hairspray, nail polish and some soaps.

Professor Robin Whyatt, from Columbia University's Mailman School of Public Health in New York, who took part in the study, said: "The magnitude of these IQ differences is troubling.

"A six or seven-point decline in IQ may have substantial consequences for academic achievement and occupational potential."

The scientists assessed the pthalate exposure of 328 New York City women and their children, all of whom were from low-income communities.

Break down chemicals from the compounds left after they had been processed in the body were measured in urine samples during the last three months of pregnancy.

Children of mothers whose exposure to DnBP and DiBP was in the top 25% had IQ scores 6.6 and 7.6 points lower, respectively, than those of mothers exposed to the lowest concentrations.

Associations were also seen for specific aspects of IQ, such as perceptual reasoning, working memory, and processing speed.

None of the women had been exposed to unusual levels of the chemicals, the researchers said, whose findings were reported in the online journal Public Library of Science ONE.

Lead author Dr Pam Factor-Litvak, also from Columbia University, said: "Pregnant women across the United States are exposed to phthalates almost daily, many at levels similar to those that we found were associated with substantial reductions in the IQ of children.

"While there has been some regulation to ban phthalates from toys of young children, there is no legislation governing exposure during pregnancy, which is likely the most sensitive period for brain development.

"Indeed, phthalates are not required to be on product labelling."

The scientists urged pregnant women not to microwave food in plastic containers, to avoid scented products as much as possible, and not to use certain recyclable plastics.