They say it may explain why almost all people who have peanut allergy have an eczema rash during their first six months of life. It is suggested that the allergens get in through the skin of infants with eczema, increasing their risk of developing an allergy.

"Kissing is essential for a child's development and we do not advocate avoidance of kissing, but caution against high-saliva-volume kisses soon after peanut consumption in infants with eczema," they say in a letter in the Journal of Allergy and Clinical Immunology.

In the past 20 years the number of children diagnosed with peanut allergy has nearly doubled. Currently one in 70 children in the UK is affected.

Dr George Du Toit, who is involved in the Learning Early About Peanut Allergy Study (Leap), said that kissing was only one of many possible routes of environmental peanut exposure. "There are other important mechanisms such as handling of the infant with peanut-contaminated hands. Traces of peanut may still be found on hands and table surfaces even after washing."

Hidden danger

Peanuts may be present in a surprising range of foods, including fruit yoghurts, chilli con carne, margarine, packet soup, salad dressings, cakes, biscuits and ice cream