The UK will be the biggest country in Western Europe by 2050, overtaking both France and Germany, figures showed.
The UK's population will increase to 77 million in just 40 years' time from 62.2 million today, a rise of almost 24%, the US-based Population Reference Bureau projected.
The surge would make it bigger than both France and Germany, where populations are predicted to rise to 70 million and 71.5 million respectively.
The population of Western Europe as a whole will remain fairly constant - 190 million in 2050 compared with 189 million today - while figures for the whole of Europe suggest the population will drop from 739 million to 720 million.
But the world's population is set to shoot up, rising to 9.49 billion from 6.89 billion, the report showed.
Much of this increase will come from India, which will overtake China to become the largest country in the world as its population swells to 1.75 billion from 1.19 billion.
China's population will also increase, up from 1.34 billion to 1.48 billion, the report showed.
Figures released last month by the Office for National Statistics showed the UK's population was up 0.6% in 2008/09 on the previous year, with 123,000 more births than seven years earlier.