At least 2.2 million migrants will arrive in the rich world every year from now until 2050, the United Nations has said
Britain's population will rise from 60 million to approaching 69 million by 2050 - almost entirely because of immigration.
The latest figures from the UN's population division predict a global upheaval without parallel in human history over the next four decades.
There will be billions more people in Africa, Asia and the Middle East. Of these, tens of millions will migrate to Europe and America, while the indigenous populations of most countries in the rich world will either stagnate or decline.
In total, the world's population will grow by 2.5 billion and reach about 9.2 billion by 2050.
This increase - almost all of which will occur in Africa, Asia and the Middle East - is the equivalent of the global population in 1950.
While some countries will grow exponentially, others will shrink dramatically.
The UN predicts the steady depopulation of vast areas of eastern Europe and the former Communist world, as a result of high levels of emigration and birth rates running persistently below replacement levels.
Bulgaria's population will fall by 35 per cent by 2050. Ukraine's will plummet by 33 per cent, Russia's by one quarter and Poland's by one fifth. There will be 10 per cent fewer Germans and seven per cent fewer Italians.
But the flow of migrants across borders will dramatically increase the populations of other developed countries.
"The population of the more developed regions is expected to remain largely unchanged at 1.2 billion, and would have declined, were it not for the projected net migration from developing to developed countries," said the UN.
The level of sustained, mass migration across borders that the world will experience over the next four decades is unprecedented.
Between 1970 and 1980, the rich world took about one million migrants a year from poor countries. During the next 43 years, immigration will run at more than twice that level and approach 2.3 million every year from now until 2050.
Of these migrants, some 400,000 will leave Africa every year and about 1.2 million will emigrate from Asia. The gap in wealth and opportunity between the rich and poor worlds will be the most significant "pull factor" behind this change. But the pressure exerted by rapidly rising populations in developing countries will also be an important underlying cause.
By 2050, India will have the highest population in the world, totalling almost 1.7 billion people. There will be 292 million Pakistanis, giving their country the fifth biggest population. Nigeria will have 289 million people - making it the world's sixth most populous country - and Uganda's population will rise to 93 million, comfortably exceeding the totals in both its larger neighbours, Kenya and Tanzania.
This massive population growth will lead to land degradation on a huge scale and place an immense strain on the limited water resources of poor countries. Malawi cannot feed its present population of 13 million - and every year its soils become more degraded and yields steadily fewer crops.
By 2050, the UN forecasts that it will have almost 32 million people - more than twice as many as today. Population growth on this scale will almost certainly leave Malawi permanently dependent on international food aid to keep millions of its people alive.
The UN's population predictions have proved largely accurate in the past. While the margin of error for these figures runs into the millions, the broad trends they disclose are undisputed.
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