Eating peanut products as a baby dramatically cuts the risk of allergy, a study 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.
Specialists said the findings could apply to other allergies and may change diets around, but warned parents not to experiment at home.
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 Jewish children in the UK.
The trial, reported in the New England Journal of Medicine, 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.
By James Gallagher, health editor, BBC News website
Allergy levels are soaring.
In the US, the prevalence has more than quadrupled since 2008, and it's a pattern replicated across much of the Western world as well as parts of Asia and Africa.
This study has generated huge excitement at what it could mean for preventing allergies developing.
However, there are still many unanswered questions.
How regularly do children have to take the peanut snacks? What stage should they start? What happens when the children stop taking the peanut snacks?
How are health care services going to adopt this?
And excitingly, will this work in other types of food allergy?
It is these unknowns that mean no doctors are saying parents should take matters into their own hands.
But as a New England Journal of Medicine editorial states the results are "so compelling and the problem of the increasing prevalence of peanut allergy so alarming, new guidelines should be forthcoming very soon".
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."
When to eat them?
- There is often confusion about when peanuts are safe as the guidelines used to advocate avoidance
- Peanuts are now thought to be safe in pregnancy
- If there is no family history of allergies or eczema then health officials say peanut butter and other ground or crushed nuts are okay after six months
- If there is a heightened risk then parents should consult a doctor.
- This research suggests careful introduction of peanut may help such children, but parents should not do this on their own
- No child under five should eat a whole nut
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."