Mothers have been urged to breast-feed for longer after new research indicated that it improves child intelligence and language ability.

Scientists found that greater duration of breast-feeding, even when it was not exclusive, led to higher intelligence scores at age seven.

Compared with bottle-fed children, seven-year-olds breast-fed for the first year of life were likely to score four points higher in a test of verbal IQ.

Verbal intelligence scores at age seven increased by 0.35 points for every extra month of breast-feeding. A similar but less pronounced trend was seen for non-verbal IQ scores, which increased by 0.29 points per breast-feeding month

Three-year-olds also benefited, with those who had been breast-fed longer having higher scores in a picture vocabulary language acquisition test. Exclusive breast-feeding – rather than any level of breast-feeding – had the greatest effect, boosting verbal IQ scores by nearly a point per month.

Seven-year-olds who were exclusively breast-fed for the first six months after birth had an almost five point advantage over those who were bottle fed.

A US team of researchers reported the findings in the journal JAMA Pediatrics.

The scientists, led by Dr Mandy Belfort, from Boston Children's Hospital, wrote: "These findings support national and international recommendations to promote exclusive breast-feeding through age six months and continuation of breast-feeding through at least age one year."

The study included 1312 mothers and children.

Certain nutrients in breast milk may benefit the developing infant brain, it has been suggested. One of these is docosahexaenoic (DHA), which is abundant in fish. Part of the research looked at whether mothers' fish consumption was linked to the benefits of breast- feeding. However, the results were non-significant.

In an accompanying editorial, Dr Dimitri Christakis from Seattle Children's Hospital Research Institute, wrote: "The problem currently is not so much that most women do not initiate breast-feeding, it is that they do not sustain it.

He added that breast-feeding in public should be "destigmatised".