Eating peanut products as a baby dramatically cuts the risk of allergy, a study reported in the New England Journal of Medicine suggests.
Trials on 628 babies prone to developing peanut allergy found the risk was cut by over 80%.
The King's College London researchers said it was the "first time" that allergy development had been reduced.
But specialists warned at-risk families should not experiment with peanut products without medical advice.
The research team in London had previously found that Jewish children in Israel who started eating peanuts earlier in life had allergy levels 10 times lower than their equivalents in the UK.
The trial focused on babies as young as four months who had already developed eczema - an early warning sign of allergies.
Skin-prick tests were used to identify those who had not yet developed peanut allergy or had only a very mild response.
Children under five should not eat whole peanuts, because of the risk of choking, so half were given a peanut-based snack. The other half continued avoiding peanuts.
The trial indicated that for every 100 children, 14 would normally go on to develop an allergy by the age of five.
But this fell by 86% to just two out of every 100 children with the therapy.
Even the children who were already becoming sensitive to peanuts benefited. Their allergy rates fell from 35% to 11%.
Lead researcher Prof Gideon Lack told the BBC: "[It was] exciting to us to realise for the first time that in allergy, we can actually truly prevent the development of disease.
"It represents a real shift in culture."
He said that high-risk children "need to be evaluated, have skin-prick testing and dietary advice, [before], in most cases, early introduction of peanut".
Prof Lack added: "We realise this goes very much contrary to previous advice, but it is very much essential that we direct our attention to this group of infants and stem this growing epidemic of peanut allergy."
Until 2008, at-risk families were told to actively avoid peanut products and other sources of allergic reactions.
The findings have attracted excited responses from other doctors, and there is speculation similar approaches might work with other allergies, such as egg protein.
Prof Simon Murch, from University Hospital Coventry, a spokesman for the Royal College of Paediatrics and Child Health, said the results were "brilliant".
He said: "It is potentially a very significant moment as it demonstrates that turning around our current approach may give better results.
"Obviously more studies will have to be performed on other potential allergens, but it is a very significant paper and is likely to change practice, at least for peanut, around the world."
He added that it was "unlikely" that previous policies were behind the rising tide of allergies as cleaner homes, modern food and gut bacteria were also likely to be playing a role.
However, he cautioned families not to rush out and buy a jar of peanut butter.
"This has to be looked at very closely by regulatory bodies, it is very possible this will lead to changes in recommendations," he said.
"But this is a single study, although very well conducted, so it is absolutely not at the stage you can recommend to families."
Dr Andrew Clark, a leading allergy specialist at Addenbrooke's Hospital in Cambridge, found last year that allergic children could be trained to tolerate peanut by slowly exposing them to higher and higher doses.
He said: "This could be a turning point in the way we try to prevent food allergy in the future, it really does prove a principle that it is possible to reduce the prevalence of peanut allergy early in childhood by feeding infants peanut in a careful and controlled way.
"But we should remember this study was carried out at an internationally renowned centre, and they selected children at quite low risk of a severe reaction.
"Don't try this at home."