Breast-feeding babies gives them the best start in life, but too much of a good thing may make them more prone to allergy, a study has found. Babies fed exclusively on mother’s milk for up to six months are known to be at reduced risk of allergies such as eczema and asthma. But a study shows extending exclusive breast-feeding beyond six months may increase the risks.

The research was started 20 years ago when scientists at the Helsinki Skin and Allergy Hospital in Finland asked 200 mothers to breast-feed their newborn children for as long as possible.

The children were assessed for allergies at the ages of five, 11 and 20. Feeding children exclusively on breast milk for nine months or more seemed to increase their risk of allergic conditions such as eczema and food hypersensitivity, according to New Scientist magazine.

At age five, more than half of children with a family history of allergy who had been breast-fed for at least nine months were showing allergic symptoms compared with one fifth of those who had been breast-fed for between two and six months. The researchers noted that children who developed allergies after prolonged exclusive breast-feeding were most likely to do so during the first years.

This suggested that environmental factors such as pollen exposure, diet and disease played a more important role in the onset of allergies in later childhood and adulthood. Over-exposure to breast milk may provide too much early protection against these triggers.

“A beautiful hypothesis is that there is a time window when the immune system needs to be exposed to external antigens [foreign proteins] for it to develop properly,” one researcher said.

Rosie Dobbs, a policy researcher at the National Childbirth Trust, said few babies were at risk because figures showed fewer than 2 per cent in Britain were exclusively breast-fed at six months. “Most babies help themselves from their parents’ plates by six or seven months,” she said.