Scientists at the University of Groningen in the Netherlands studied school results of 570 nine-year-old children who had been born in a Dutch hospital between 1975 and 1978 and whose mother had smoked during the pregnancy.

They found that only children who had been bottle-fed performed poorly on the school tests.

"Our results indicate that negative effects of maternal smoking on children's cognitive performance were limited to those who had not been breast fed," Dr Laura Batstra said in a report in the Journal of Epidemiology and Community Health.

Batstra and her colleagues said psychological or other factors could be involved but suggested that ingredients in breast milk may promote brain development and counteract the negative effects of maternal smoking.

"This notion has important practical implications, especially for nicotine-addicted future mothers," Batstra said.

"Apart from helping these mothers to stop or diminish their tobacco consumption, they should be encouraged to breast feed," she added.

Medical studies have shown that breast feeding offers tremendous benefits for babies. It reduces their risk of ear infections, allergies, vomiting and diarrhea .

Norwegian scientists have also found a positive association between the duration of breast feeding and the child's mental development. The longer the child was breast fed, the greater the benefit, even after adjusting for maternal age, education, intelligence and smoking at the time of conception.