Relying purely on breastfeeding for the first six months might not be best for babies, experts in the UK have warned.the team said breastfed babies may benefit from being given solid food earlier.
Current advice suggests weaning should occur at six months, but the UCL team say it could happen as early as four.
They suggest later weaning may increase food allergies and iron deficiency levels, but other experts backed the existing guidance.
Ten years ago, the World Health Organization published global advice advocating babies be exclusively breastfed for six months.
The research team, led by Dr Mary Fewtrell a paediatrician from the University of London Institute of Child Health, said it supported the recommendation for developing countries, where access to clean water and safe weaning foods is limited, and there is a high risk of infant death and illness.
But they added: "Many western countries, including 65% of European member states and the US, elected not to follow this recommendation fully, if at all.
But in 2003, a health minister said the UK would comply.
Parents 'cannot win' The WHO recommendation "rested largely" on a review of 16 studies, including seven from developing countries.
It concluded that babies just given breast milk for six months had fewer infections and experienced no growth problems.
End Quote Dr Mary Fewtrell ResearcherThere are a lot of babies being weaned before six months anyway”
But another review of 33 studies found "no compelling evidence" not to introduce solids at four to six months, the experts said.Some research has also shown that six months of breastfeeding does not give babies all the nutrition they need.
A US 2007 study found there was an increased risk of anaemia compared with those introduced to solids at four to six months.
Swedish research also found that the incidence of early onset coeliac disease increased after a recommendation to delay introduction of gluten until age six months, but fell back after the recommendation reverted to four months.
Dr Alan Lucas, director of the Institute of Health, said: "The WHO recommendation is very sensible for developing countries.
"But in the UK, it's important we take a balanced look at the evidence."
Dr Fewtrell added: "When you look at the figures, there are a lot of babies being weaned before six months anyway - and that's probably the most important thing in terms of hard evidence."