Researchers looked at almost 14,000 children for more than six years and found that those who were breastfed did significantly better in IQ tests.
Previous research has suggested that breastfeeding promotes IQ, but the results have not been clear cut.
For the new study, a team of Canadian led scientists carried out a randomised breastfeeding trial involving patients at 31 maternity hospitals and clinics in Belarus.
Half the mothers were enrolled in a programme which encouraged long term exclusive breastfeeding. The rest received normal care and were allowed to choose whether or not to breastfeed.
The study was designed not to be influenced by factors such as the mother’s intelligence or the way she interacted with her baby.
Children’s mental ability was assessed by IQ tests administered by doctors and teachers’ ratings of their performance in reading, writing, mathematics and other subjects.
At the age of six and a half, exclusively breastfed children scored 5.9 points higher on average in tests of overall intelligence.
They also scored 7.5 points higher in tests of verbal intelligence, and 2.9 points in tests of non-verbal intelligence.
The teachers gave children on the breastfeeding programme a significantly higher rating in both reading and writing.
Belinda Phipps, chief executive of the National Childbirth Trust (NTC), which provides counsellors to help women with breastfeeding, said people “shouldn’t be surprised” by the results.
Ms Phipps added: “We welcome this paper because it is another piece of evidence which shows the benefits of breastfeeding.
“We now need research to show whether [the rise in intelligence] is to do with breast milk or the act of breastfeeding.”