Breast may be best but failing to give babies some solid foods before the age of six months could lead to iron deficiency and make them prone to allergies, according to a new study.
The fresh evidence contradicts World Health Organization guidance leaving breast feeding campaigners outraged and mothers confused.
Scientists are now suggesting that current advice to mothers to breastfeed exclusively for the first six months of a baby's life should be dropped in favour of a four-month rule.
Experts led by a paediatrician from University College London's Institute of Child Health say babies could suffer iron deficiency and may be more prone to allergies if they only receive breast milk.
This flies in the face of advice from the WHO which announced in 2001 that exclusive breastfeeding for six months would give a baby the best start in life.
Today, in the British Medical Journal, doctors from several leading child health institutes say the evidence for the WHO guidance was never there -- and that failing to start weaning babies on to solids before six months could be harmful.
They said babies who are exclusively breastfed for the first six months have a greater chance of iron deficiency anaemia "known to be linked to the irreversible adverse mental, motor or psychosocial outcomes".
The researchers also suggest that these babies may have a higher incidence of food allergies. The third potential risk is coeliac disease. The scientists cited Sweden, where numbers of children developing coeliac disease rose after advice to mothers to delay the introduction of gluten into their child's diet until after six months.
It fell when the recommendation reverted to four months.
Institute expert Mary Fewtrell said: "Exclusive breastfeeding protects against infections, which is critical in developing countries, but less important where hygiene and sanitation are better."