Supplements may be required

They are concerned that children are no longer exposed to sufficient sunlight to generate healthy levels of the vitamin themselves.

The warning was issued by the American Academy of Pediatrics in its journal Pediatrics.

Rickets is rarely fatal but can soften bones and result in bowing of the legs and impede growth.

It is also linked to respiratory and heart problems.

The number of cases of rickets in the US has risen in recent years. A similar trend has been seen in the UK.

Breast milk contains many valuable nutrients but not enough Vitamin D to meet the daily requirement of 200 International Units.

Exposure to the sun's rays normally generates Vitamin D in the skin, but applying sun block stops that process.

Dr Lawrence Gartner, of the University of Chicago, and one of the authors of the policy statement, said: "The normal source of Vitamin D for a baby is sunshine.

"The problem is we can no longer recommend that babies be put out in the sun because of the risk of skin cancer, which has increased significantly in the past several years, probably because of the diminishing ozone layer."

Baby formula contains Vitamin D but breast-fed babies likely need a liquid supplement, which is squirted into the baby's mouth with an eyedropper.