Research shows that the first bacteria newborns come into contact with play a major role in determining lifelong health.

Babies born naturally are exposed to a range of beneficial bacteria while moving down the birth canal. But children who are delivered by caesarean are not exposed to these microbes, putting them at risk of weight gain, asthma and immune disorders.

Professor Tim Spector, of King’s College London, said more awareness is needed of the ‘downsides’ of caesareans – which now make up one quarter of births in Britain.

Speaking at the Royal College of Obstetricians and Gynaecologists’ conference in London, he said: ‘Every mammal gets its microbes when they pass through the birth canal.’

He said that obesity is not determined by calorie intake but by these microbes, adding: ‘The more diverse microbes you have inside you, the healthier you are.’