The world's population will stabilize in 300 years to about nine billion people, with an average life expectancy of 95 years, the United Nations predicts.

In Japan, where people already tend to live longest, the average person will hang on to 106 in 2300, says a UN report released Thursday.

The report departs from dire predictions in recent years that the world's population could reach 12 billion by 2300.

The average woman will have two children in coming years, raising the world population to 9 billion in 2300 from its 6.4 billion now, the UN's Population Division said.

Long-term estimates of population growth dropped because of declining fertility rates.

The report also forecasts that India will pass China to become the world's most populous country.

It will be followed by China and the United States, and Africa will have twice as many people – about 25 per cent of the global total.

The average age is expected to jump to 50 years from 26.

Its authors report warn that the slightest changes in their model, such as a tiny drop in fertility rates, could dramatically alter the results.

The report says that if women kept having as many children as they do currently, the global population would skyrocket to 1.34 trillion in 2300, a completely unsustainable level.