Breastfeeding could have a direct impact on intelligence and a high IQ, a long-term study suggests.

According to the BBC, the research in Brazil traced nearly 3,500 babies, from all walks of life, and found those who had been breastfed for longer went on to score higher on IQ tests as adults.

Experts say the results, which was published in The Lancet Global Health, while not conclusive, appear to back current advice that babies should be exclusively breastfed for six months.

The findings however stressed that there are many different factors other than breastfeeding that could have an impact on intelligence.

Dr Bernardo Lessa Horta, from the Federal University of Pelotas, Brazil, said his study offers a unique insight because in the population he studied, breastfeeding was evenly distributed across social class.

Most of the babies, irrespective of social class, were breastfed - some for less than a month and others for more than a year, with those who were breastfed for longer scoring higher on measures of intelligence as adults.

They were also more likely to earn a higher wage and to have completed more schooling.

Dr Horta believes breast milk may offer an advantage because it is a good source of long-chain saturated fatty acids which are essential for brain development.

But experts say the study findings cannot confirm this and that much more research is needed to explore any possible link between breastfeeding and intelligence.

Kevin Fenton, national director of health and wellbeing, Public Health England, said there was strong evidence that breastfeeding provides some health benefits for babies such as reduced respiratory and gastrointestinal infections in infancy.

He therefore advised exclusive breastfeeding for around the first 6 months of life.

While Dr Colin Michie, chairman of the Royal College of Paediatrics and Child Health's nutrition committee, said what made this particular study powerful was the fact that looks at a number of other factors including education achievement and income at age 30 which, along with the high sample size.