India is set to overtake China as the world's most populous nation by 2050, while some countries will shrink by nearly 40%, according to new research.
The Population Reference Bureau (PRB) says the next half century will see wild swings in population sizes.
It predicts that the number of people on Earth will reach 9.3bn by 2050, compared with 6.3bn today.
Britain's population is likely to overtake that of France, while the US will grow by nearly 50%, it says.
The Washington-based PRB says the general trend will be for Western developed nations to decline slightly in numbers - the US being the major exception - while developing states continue to expand rapidly.
The organisation says that at present "nearly 99% of all population increase takes place in poor countries".
India is expected to grow from 1.08bn to 1.63bn people, overtaking China, which is forecast to reach 1.44bn from 1.3bn currently.
The US will remain the third biggest nation, according to the report, growing to 420m from 294m people.
Britain is expected to grow only slightly, to 65m, from 59.5m, while many of its European neighbours decline.
In Eastern Europe the decline will be marked, if current trends continue.
Bulgaria could lose 38% of its 7.8m inhabitants, with Russia declining by 17% - some 25m people.
Anomalies affect prediction
The projections are based on infant mortality rates, life expectancy, fertility rates and age structure, as well as factors such as contraception and Aids rates.
What the study cannot predict is how migration between nations may affect population growth.
Carl Haub, the chairman of population information at the PRB, admits it is not possible to know exactly how the world will grow. "So many demographic anomalies exist that the future is uncertain," he said.
Most recent population studies agree, however, that humanity will grow rapidly, at least in the near future, and that the planet's resources will be increasingly stretched.
The UN published a recent study, whose "medium-case" scenario was that the world would reach 9bn by 2300 - 250 years later than PRB predict.
In March the US Census Bureau said world growth was actually slowing, and that Aids meant Africa's population might actually begin to decline.