Fifteen million babies, one in 10 births, are born prematurely every year, a global project suggests.
One million of these babies die soon after birth. The joint report, led by the WHO, says three quarters of deaths could be prevented with basic care.
For the first time premature birth rates have been estimated by country, with the highest risk being in Africa.
In the UK about 8% of babies are born too soon and this rate is rising partly due to obesity and later motherhood.
There are nearly 60,000 premature births every year in the UK.
Andy Cole, from premature baby charity Bliss, said "it is worrying that the UK's preterm birth rate is significantly higher than countries such as Sweden, Norway and Ireland, and highlights the need for well-co-ordinated and high-quality antenatal care for all women identified as high risk".
A premature or preterm baby is one that is born before 37 weeks after the first day of the mother's last period. A full term baby is when pregnancy lasts 40 weeks.
Forty-four organisations contributed to The Born Too Soon report , which estimated premature birth rates - the number of babies born too soon, out of the total of number of live births - for 184 individual countries, in the first study of its kind.
Of the 11 countries where over 15% of babies are born too early, all but two are in sub-Saharan Africa. However, the report highlights it is a problem worldwide.
"It is very striking to see that preterm births have a similar burden all around the world - but due to different reasons," Dr Lale Say from the WHO said.
"In developing countries it is due to things like infections, HIV, malaria and poor nutrition.
"In developed countries there are totally different risk factors - an older delivery age, diabetes, obesity and multiple births due to IVF."
The report also mentions caesarean sections before full term, which are not always medically needed, as a reason for increasing premature baby rates.
A leading cause of death
Dr Joy Lawn, co-editor of the report and Director for Save the Children said: "This report shows the problem is much bigger than expected or realised. Being born too soon is an unrecognised killer."
In children under five, prematurity is the leading cause of death worldwide, after pneumonia. Many premature babies that do survive develop learning difficulties and visual and hearing problems.
"This has been a shocker to those who work in child survival programmes - people were almost falling off their chairs when I reported back our findings," said Dr Lawn.
"We have been working on problems identified 20 years ago. There has been progress in pneumonia, and diarrhoea as a cause of death has seen a major drop, but preterm birth has not been on anyone's 'to do' list."
"There is no excuse for 80% of babies, who are less than eight weeks early, to die - it's lack of food and warmth, not lack of intensive care."
Experts at the UN say simple and inexpensive care, like antiseptic cream to prevent cord infection, one US dollar (60 pence) steroid injections given to mothers to help foetal lung development, and antibiotics to fight infection, can help keep premature babies alive.
They also advocate the use of kangaroo care - where the baby is tied, skin to skin, on the mothers' front - which reduces infection, keeps the baby warm and makes it easy to breastfeed. This has been proven to dramatically reduce newborn death.
Not surprisingly, there are big inequalities in survival rates around the world.
This is highlighted in the US - as, while it is makes the top ten for the highest number of premature births in the world, it is only number 37 for deaths - because of very effective and expensive intensive care.
Dr Christopher Howson, from March of Dimes, a baby charity which collaborated on the study said: "In low income countries, more than 90% of extremely preterm babies die within the first few days of life, while less than 10% die in high income countries."
"However, this is a solvable problem. A number of countries, for example, Ecuador, Botswana, Turkey, Oman and Sri Lanka have halved their neonatal deaths from preterm birth through improving [care for] serious complications like infections and respiratory distress."
It is hoped the report will spotlight premature births, especially in sub-Saharan Africa, and make it an urgent priority to help reach the UN Millennium Development Goal 4 set in 2000 - which calls for the reduction of young child deaths by two-thirds in 2015.