The population of the UK is set to increase by 4.4 million to 65 million by 2016, according to new projections.
The Office for National Statistics (ONS) estimates that 2.1 million of the overall rise can be put down to immigration alone.
Further projections say the population would reach 71 million by 2031 and 77 million in 2051.
Forecasts of fertility, life expectancy and inward migration have all increased since they were last made in 2004.
Estimates of the amount of migration inflow have risen to a net 190,000 a year from 145,000 a year.
In percentages, total population rises by 2016 are put at 8% for England, 7% for Northern Ireland, 5% for Wales and 3% for Scotland.
The number of those aged 75 and over is projected to increase from 4.7 million in 2006 to 5.5 million by 2016 to 8.2 million by 2031.
The projection says more births than deaths will account for just over half of population growth to 2031.
Detailed figures however show some of those born to relatively younger migrants will push up the birth rate.
Experts say this means 69% of Britain's predicted population growth maybe associated directly, or indirectly, to immigration.
Conservative spokesman Damian Green said it was "ever more urgent" for the government to control immigration.
He called for a "gradual and sensible growth of population rather than this headlong growth in population which is bound to put extra strain on public services such as provision of housing and education".
Immigration Minister Liam Byrne said changes were being made to the immigration system in the next 12 months which would mean "only those that Britain needs can come to work and study".
Sir Andrew Green, chairman of pressure group Migrationwatch, said "Population growing at this speed is just incredible - twice the population of London by the mid-century
"Huge impact on our infrastructure, on our public services, and indeed on the whole nature of our society, and all of it taking place without the public ever being consulted."
A spokeswoman for the Joint Council for the Welfare of Immigrants said: "We need figures - not just the growth of population but where it is growing and how, and the impact on services and the economy.
"Just calling for a cap on immigration will not respond adequately to complex changes in the existing population such as the growth of single-person households and the migration of the existing population within the UK."
Jonathon Porritt, patron of the Optimum Population Trust think-tank, called for an "intelligent debate" on growing numbers of people in the UK.
He said: "If it quickly defaults into pro or anti-immigration... it means we constantly avoid the discussion about human numbers.
"We miss the subtle debate about the impact of population on an already congested island."
BBC News economics editor Evan Davis said if the UK tried to shoehorn the population into existing roads, hospitals and schools it could lead to tensions and feelings of crowdedness.
He said he saw the figures as a wake-up call to think about how the process of absorbing population growth was managed.