Four in 10 maternity units in England are failing to do enough to promote breastfeeding, according to a report.
A study for Unicef UK said the health of mothers and babies was being put at risk by the continued failure to implement minimum standards of care.
Rates of breastfeeding in the UK are among the lowest in Europe.
Breastfeeding has been shown to protect babies against a range of illnesses and help guard women against breast and ovarian cancer.
Last year, the National Institute for Health and Clinical Excellence (Nice) said the NHS should implement Unicef's Baby Friendly Initiative (BFI) as a "minimum standard".
The initiative aims to support mothers through encouraging close contact, training staff and advising on topics such as expressing milk and helping a baby "latch on".
According to Unicef UK, many midwives and health visitors have had little or no training in how to breastfeed.
It published details showing that 38% of hospitals in England were failing to take up the initiative, which also supports mothers in bonding with their baby if they choose not to breastfeed.
A spokeswoman said it was possible that some of those hospitals could be implementing parts of the Nice guidelines but this was not being externally assessed.
"Women and babies could be falling through the net," she said.
