Pregnant women who have a Caesarean should be warned it could almost double their baby's chance of developing asthma, say researchers.

Surgical deliveries alter the way the immune system develops by delaying exposure to gut bacteria, making infants more vulnerable to allergies.

A study of 2917 births found C-section babies were nearly 80% more likely to be diagnosed with asthma. The risk almost trebled for those with two allergic parents, giving them a genetic predisposition to the condition.

Dr Caroline Roduit, of the University Children's Hospital, Zurich, said: "The increased rate of Caesarean section is partly due to maternal demand without medical reason.

"In this situation the mother should be informed of the risk of asthma for her child, especially when the parents have a history of allergy or asthma."

Her team monitored the respiratory health of the children until they reached the age of eight, by which time 362 (12.4%) were found to be asthmatic and prescribed inhaled steroids.

Even though only 247 (8.5%) of the children had been born by C-section they were 79% more likely to have been diagnosed with asthma than their naturally delivered peers.

The association was even stronger for the 9% of children with two allergic parents - being almost three times more likely to be asthmatic at eight years old.

Dr Roduit, whose findings are to be published in Thorax, said rates of asthma have soared in industrialised countries in parallel with a rise in C-section births, most of which are carried out for cosmetic purposes.

She says the link between C-section births and an increased risk of asthma may lie in the timing of the priming of the immune system, with surgical deliveries delaying exposure to microbes.

Previous research has suggested gut bacteria play a key role in the development of the immune system and it may be that naturally delivered babies pick up vital bugs from the mother, whereas babies born by C-section merely acquire bacteria from the hospital environment.

Earlier studies investigating the link between asthma and C-section births have produced conflicting results.

Latest figures show 23% of babies delivered in Britain's hospitals in 2005/2006 were born by C-section - more than twice that recorded 15 years ago.