Giving babies smooth peanut butter between the age of 4 and 6 months could cause peanut allergies to plummet, scientists say.
Researchers found there was a clear "window of opportunity" to expose babies to peanuts and reduce the risk of them developing an allergy.
It needs to be done by the time they are six months old, scientists found - and even earlier, at four months, for babies with eczema, which is a risk factor for allergy.
Introducing peanut products into all babies' diets by six months could reduce peanut allergy across the population by up to 77%, the study found.
But waiting to introduce the peanut products until a baby's first birthday would lead to only a 33% reduction.
Most peanut allergies have already developed by the time a child turns one.
Peanut allergy affects around 2% (1 in 50) of children in the UK and has been increasing in recent decades, according to Allergy UK.
The study stresses that the baby should be developmentally ready to start solids and smooth peanut butter or baby-friendly snacks should be given - not whole or broken peanuts.
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The study, which was published in the Journal of Allergy and Clinical Immunology Now, found the greatest benefit could be achieved if the whole population was targeted.
The University of Southampton's Professor Graham Roberts, who led the study, said advice to avoid peanuts had "understandably led to parental fear of early introduction".
But encouraging parents to introduce their babies to peanuts could be a "simple, low-cost, safe intervention ... that would deliver vast benefits for future generations".
Professor Gideon Lack from King's College London said the study reflects the experience in Israel, where peanut snacks are given to young children and peanut allergies are rare.