The world's population is expected to rise from the current 6.5 billion to 9.1 billion by 2050, the UN says.
Virtually all the growth will be in the developing world, according to a report by the UN Population Division.
By contrast, the population of developed countries will remain almost static at 1.2 billion, the report adds.
It says India will overtake China as the world's most populous country by 2030 - five years earlier than previously expected.
The figures in the revised report are based on national censuses, population surveys and review of trends.
In 2002, the UN Population Division had estimated a population in 2050 of 8.9 billion.
The new report predicts that the population in the world's 50 poorest countries will more than double by 2050.
It says that nations such as Afghanistan, Chad and East Timor will see their numbers going up three-fold.
"They are the ones not being able to provide adequate shelter, adequate food for all their people," Haina Zlotnik, the UN Population Division's chief, told a news conference in New York.
"If fertility dropped downwards, they would be buying time to face the problems they are facing," Ms Zlotnik said.
The report also says that Africa - unlike other regions - has seen average life expectancy at birth decline from 62 years in 1995 to 48 years in 2000-2005.
It attributes the sharp fall to the continuing spread of HIV/Aids and other infectious diseases, as well as armed conflicts and economic stagnation.
However, the overall trend shows a lower rate of growth in the past 20 to 50 years.
"The population continues to grow but at a lower pace," said Thomas Buettner, author of the report.
"Family planning and lower fertility make the difference," he said.
Fertility is expected to decline from 2.6 children per woman today to slightly over 2 children per woman in 2050, the report says.
It is going to be a strain on the world
Haina Zlotnik, UN Population Division chief