In total, the world's population will grow by 2.5 billion and reach about 9.2 billion by 2050.

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.