The number of people on Earth is set to shrink for the first time since the Middle Ages, experts say.
Forecasts published in the Lancet predict the world’s population will increase from 7.8 billion now to 9.7 billion in 2064, before falling to 8.8 billion by 2100.
The major research project funded by Microsoft founder Bill Gates analysed the consequences of fewer people having children.
A Washington University-led team found 23 countries including Japan, Thailand, Italy and Spain forecast their populations more than halve.
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Fewer working-age people will shift economic power away from major European and Asian nations towards Africa and India.
The landmark research also found immigration will be vital if Britain is to remain a leading economy by the end of the century.
The UK economy is predicted to slip from 6th to 7th for Gross Domestic Product by the end of the century.
Its population will increase from 66.6 million to peak at 74.9 million in 2063, before falling to 71.5 million by 2100.
However, Italy’s population is predicted to fall from a peak of 61 million in 2014 down to 30.5 million in 2100.
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In Spain it has peaked at 46 million in 2017 and is expected to steadily decline to 23 million, experts say.
Spain will then slip from 13th position by GDP to 28th while Italy will fall from 9th to 25th.
China will replace the US as the global power with highest GDP in 2035, before the US potentially regains the top spot in 2098 “if immigration continues to sustain the workforce”.
China’s population - currently 1.41 billion - will start to fall from mid-century and will almost halve to 730 million by 2100, the researchers claim. It will soon peak at 1.43 billion in 2024.
Dr Richard Horton, Editor-in-Chief of The Lancet, adds: “This important research charts a future we need to be planning for urgently.
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“The 21st century will see a revolution in the story of our human civilisation. Africa and the Arab World will shape our future, while Europe and Asia will recede in their influence.
“By the end of the century, the world will be multipolar, with India, Nigeria, China, and the US the dominant powers.
“This will truly be a new world, one we should be preparing for today.”
Researchers found 183 of the 195 countries included will have fertility rates below that needed to stop the population falling.
The world population has been rising continuously since the end of the Black Death, around the year 1350.
It exploded during the 20th century from 1.6 billion in 1900 to 7.7 billion by November 2019.
People are having fewer babies as they move to cities alongside increasing access to contraception and increasing women’s rights.
Net migration to the UK is expected to remain at around 185,000 until 2050 before dropping off to 116,000 in 2100.
By comparison in Spain net migration is expected to plummet from 47,300 to 11,900 in 2100. In Italy the fall will be from 75,500 down to 29,500.
Immigration boosting the UK’s working age population will enable it to remain at just under 40 million.
By the end of the century Nigeria will be the rising power as its population surges giving it the ninth biggest economy by GDP.
Its working age population will be second only to India and followed by China then the US.
The population in the Middle East will increase and it will treble in sub-Saharan Africa from one billion to three billion.
Co-author Prof Stein Emil Vollset said: “Population decline is potentially good news for reducing carbon emissions and stress on food systems.
“However more old people and fewer young people, economic challenges will arise as societies struggle to grow with fewer workers and taxpayers.
“Countries’ abilities to generate the wealth needed to fund social support and health care for the elderly will be reduced.”