Researchers at Queen Silvia Children's Hospital in Goeteborg, Sweden, interviewed the parents of 244 Swedish and Danish babies that had died from sudden infant death syndrome (SIDS), as cot death is known, and those of 869 babies who were alive and well.

The longer the infants were fed by breast after birth, the better their chances of survival, they found.

For instance, a child that had been breast-fed for only a week after birth and then bottle-fed was nearly four times more likely to die from SIDS within four months than was a counterpart who had received breast milk throughout.

The researchers, who report their work in a specialist British journal, Archives of Disease in Childhood, took into account other factors that could affect the results, such as family income and smoking by the child's mother when she was pregnant.

They speculate that breast milk may prevent infection, but also stress that the children's sleeping position has been the biggest factor by far in SIDS.

Cot-death occurs when infants die from suffocation in their bed after getting into a position that impairs their breathing.

The risk period is between three and five months, when a child becomes more mobile, and able to wriggle into a potentially suffocating position.

The problem has been blamed on the lack of a "startle response" in the brain that triggers a gasp or a cry to kick-start normal respiration when breathing is impaired.

But what causes this abnormality is unclear. Smoking by the mother while pregnant is a well-known risk factor, but studies in Denmark, Scotland, England and New Zealand have delivered conflicting conclusions about whether lack of breast-feeding may also be a cause.

Doctors advise that a child should be placed on its back to sleep to help prevent cot death.