According to the United Nations' latest World Population Prospects, released on Thursday in New York, there will be 1,395m people in India in 2025, and 1,593m in 2050. In China the population will grow to 1,441m by 2025, before dropping back to 1,392m in 2050.

“We've been saying for a while that India would cross over China before 2050, but the crossover has been getting earlier and earlier, and we now say it will happen before 2030 (not including Hong Kong). This is five years earlier than we said two years ago,“ Cheryl Sawyer, a UN demographer, said.

“Based on analysis of the newest censuses we're estimating lower fertility for China, while India's is slightly higher than we estimated in the past.”

In 2050, the world's population will be 9.1bn people, up from 6.5bn today, with almost all the growth registered in developing countries.

The numbers vary significantly according to differing scenarios, however, and become less certain the further they project into the future. Demographic shifts depend on fertility, mortality, and migration, which can be influenced by policy and social and economic trends.

The UN's population division said the fact that India and China will “exchange places is without doubt”, mainly because of differences in fertility. The only question is exactly when. China currently has a fertility rate of 1.7 children per woman (though rising to 1.85), while India's is just above three.

Thomas Buettner, the chief of the UN division's Estimates and Projection section, said China's changing population was due to “modernization and uprooting people from traditional lifestyles into the modern economy”, where “people have other opportunities that compete with having large families, like consumerism, travel and education.”

At the same time, “it is also due to a very rigid population policy”, although “Chinese officials have started thinking about relaxing that policy because they are concerned about rapid ageing of the population.”

Europe's population, which recently underwent a reversal in growth, is also on a downward trend. According to a medium variant, it will drop from 728m today to 653m in 2050. That figure (which incorporates Russia, but not Turkey) includes falls in Italy and Germany, although France and the UK will grow.

Japan's population declines from 128m to 112m, according to the same variant.

“The population of developed countries as a whole is expected to remain virtually unchanged between 2005 and 2050, at about 1.2bn,” the report says. “In contrast, the population of the 50 least developed countries is projected to more than double.“