Wealthy countries are not doing enough to track levels of breastfeeding within a baby's first hour of life, global health leaders have said.
A new report highlights how data on the early initiation of breastfeeding - within the first hour of life - is not recorded in many high-income countries.
Experts have suggested millions of children in high-income countries are "missing out" on the benefits of breastfeeding.
In wealthy nations, 21% of children have never been breastfed, according to a new report by the World Health Organisation and the United Nations Children's Fund (Unicef).
This compared to just 4% in low-income countries, the report adds.
"This wide gap means that 2.6 million children in high-income countries are missing out completely on the benefits of breastfeeding," the authors wrote.
They said it is not possible to report on early breastfeeding initiation rates in high-income countries.
Across the world, rates of early initiation of breastfeeding are tracked using data from household surveys.
But the authors said that while many high-income countries track breastfeeding through hospital registries or other data systems, these data are not collected using standard global indicators - such as breastfeeding initiation within the first hour of life - and are therefore not internationally comparable.
The report details early breastfeeding initiation rates across countries that do record the data, mostly low and middle-income countries.
The authors found three in five babies are not breastfed in the first hour of life.
Unicef executive director Henrietta Fore said: "When it comes to the start of breastfeeding, timing is everything. In many countries, it can even be a matter of life or death.
"Yet each year, millions of newborns miss out on the benefits of early breastfeeding and the reasons - all too often - are things we can change.
"Mothers simply don't receive enough support to breastfeed within those crucial minutes after birth, even from medical personnel at health facilities."
WHO director general Dr Tedros Adhanom Ghebreyesus, added: "Breastfeeding gives children the best possible start in life.
"We must urgently scale up support to mothers - be it from family members, health care workers, employers and governments, so they can give their children the start they deserve."
Commenting on the report Clare Livingstone, professional policy adviser at the Royal College of Midwives, said: "The evidence on the benefits of breastfeeding is incredibly strong and we a should all be doing as much as we possibly can to support women to breastfeed.
"This is particularly important for areas of the world where women many not have easy access to healthcare services or clean water.
"It is without a doubt the best way to give a baby the healthiest possible start in life, and as this report shows, starting it as soon as possible is incredibly important and can even be lifesaving."
Chris Roebuck, chief statistician at NHS Digital, said: "We have recently begun publishing breastfeeding initiation rates for most providers in England in our monthly publication on maternity services, available to view at national, regional and trust level."