The longer the child is breastfed – ideally exclusively – the higher the intelligence scores are at the age of seven.
The study also found breastfeeding can enhance language skills from the age of three.
The Duchess of Cambridge's apparent decision to try to breastfeed Prince George has been boosted by evidence showing it can raise a child's IQ
However, British experts warned that delaying the introduction of solid foods until six months at the earliest might leave some babies feeling hungry.
It emerged yesterday that the Duchess has at least one maternity dress made for breastfeeding and was given encouragement in hospital to help her baby George start on her milk.