The population of developing countries will soar unless donors give more funds to reproductive health programmes, a UN Population Fund report says.
Developing countries will see the sharpest surge
The world's 50 poorest countries will triple in size by 2050, surging to 1.7 billion people, it predicts.
Donors have been giving only half the funds pledged at a conference in Cairo in 1994, UNFPA told BBC News Online.
The money is used for programmes supporting women's rights and health care in the developing world.
UNFPA's State of the World Population 2004 report examines progress made since Cairo, when wealthy countries pledged to give an annual $6.1bn to the fund.
William Ryan, the report's author, told BBC News Online that although developing countries were making great strides to tackle the problems of a growing population, the fund was $3bn short.
"Without access to health services and education the population will continue to increase, above all in the poorest countries," he said.
US holds back
By 2050, UNFPA says, there will be 8.9 billion people sharing the planet, a slight decrease from earlier official predictions.
The US - the fund's largest donor - has blocked its donations to the body for the past three years.
The Bush administration froze $34m in funding, citing allegations that the UNFPA was involved in forced abortions in China - a charge consistently denied by the organisation.
"We note that with an additional $34m we could help provide family planning to thousands of women who need it," Mr Ryan said.
In addition to the threat of overpopulation, half a million women now die annually during childbirth in the developing world.
The fund says gaps in reproductive health care - a lack of access to information, care and practical resources like contraceptives - accounts for nearly 20% of the worldwide burden of illness and premature death.
Providing contraception for the 200 million women who want it, at a cost of $3.9bn, would avert some 52 million pregnancies each year and avoid 1.4 million infant deaths, it says.
Aids has also affected the changes in population size, the report says - the 38 African countries most affected by Aids are projected to have 823 million people in 2015.
The figure is 91 million fewer than if no Aids deaths had occurred but over 50% more than today.