Babies who are breastfed do grow up to be more intelligent as a direct result, according to a study published yesterday.

The research following the development of almost 14,000 children up to the age of six found that those of mothers who were exclusively breastfeeding had significantly higher IQs.

At the age of six and a half, youngsters who had only been fed breast milk scored 5.9 points higher on average in overall intelligence tests than those who were sometimes or always bottlefed.

They scored 7.5 points higher in tests of verbal intelligence, and 2.9 points higher in tests of non-verbal intelligence.

An array of previous research has suggested that breastfeeding promotes intelligence, but critics have long argued that the reverse is true with more intelligent mums more likely to breastfeed.

But the Canadian-led scientists who conducted the research claimed that their study had been designed to exclude factors such as the mother's intelligence.

The breastfeeding trial involving patients at 31 maternity hospitals and clinics in Belarus saw half the mothers enrolled in a programme encouraging long term exclusive breastfeeding.

The rest received normal care and were allowed to choose whether or not to breastfeed. Children's mental ability was assessed by IQ tests administered by doctors and teachers' ratings of their performance.

The teachers gave children on the breastfeeding programme a significantly higher rating in reading and writing.

Professor Michael Kramer, the study leader, from McGill University in Montreal, said: "While most studies have been based on association, we can make a causal inference between breastfeeding and intelligence because of the randomised design of our study."

The scientists, whose findings were published yesterday in the journal Archives of General Psychiatry, said it was unclear whether the benefits of breastfeeding were due to something in breast milk or related to physical and social interactions.

Possible reasons for the IQ scores included essential fatty acids and a compound called insulin-like growth factor I, found in breast milk.

On the other hand, the physical or emotional components of breastfeeding may lead to permanent changes affecting brain development, the researcher speculated.

They concluded: "Although breastfeeding initiation rates have increased substantially during the last 30 years, much less progress has been achieved in increasing the exclusivity and duration of breastfeeding.

"The consistency of our findings based on a randomised trial and those reported in previous observational studies should prove helpful in encouraging further efforts to promote, protect and support breastfeeding."

Their findings received a cautious welcome from a leading campaigner on childbirth issues in Scotland, where legislation was passed in 2005 allowing women to breastfeed in public.

Cynthia Clarkson, maternity services convener at the National Childbirth Trust in Scotland, said: "The big confounder is that mums who are older and better educated tend to be the mums who breastfeed.

"It is very difficult to deselect these factors. I would urge caution, I would not go overboard about this. They have shown the link between breastfeeding and intelligence in studies with premature babies. More research would be useful."

Regardless of whether or not breastfeeding did boost intelligence, she said the other benefits were cause enough for mums to shun the bottle.

She said: "I would promote breastfeeding and particularly support for women, because it can be very hard. It's not as natural and easy as people think."

Giving her son the best start in life

Sonia Laurence has been breastfeeding her firstborn, Matthew, since he was born 17 weeks ago.

As a midwife, she knows more than most about the general health benefits of breastfeeding. As a new mum she is keen to give her son the best start for a healthy life.

She welcomes news today that scientists have found evidence breastfeeding does promote greater intelligence.

The 37-year-old from Dunbar said: "I spend all my time teaching other women how to breastfeed and what the benefits are so it was natural that I would breastfeed too.

"I think it must have an impact on intelligence as your body is making exactly what your baby needs. Formula milk is not natural.

"Breastfeeding is good for bonding too, which I think also makes a big difference. Mostly I think the link to intelligence is a cultural effect though. There is a definite socio-economic group of women who choose to breastfeed.

"I will probably continue breastfeeding exclusively until about 20 weeks. I have been lucky, I have found breastfeeding fine. My son is a good feeder. "