The UK population could rise by as much as 10 million - nearly the size of London - by 2037 according to new official projections.
The majority of the increase attributable to migration.
Figures released by the Office for National Statistics (ONS) show a projected population increase of 9.6 million over the next 25 years from an estimated 63.7 million in mid 2012 to 73.3 million in mid-2037.
The ONS said 60% of this increase - or 5.8 million - was due to migration, either directly through new arrivals into the country or indirectly through the impact on the birth rate.
The projected 9.6 million rise in the population would mean an increase bigger than the current 8.3 million population of London, the ONS confirmed.
The figures also project an increasingly ageing population with the average age increasing from 39.7 years old in 2012 to 42.8 by mid-2037. The number of people aged 80 or older is projected to more than double over the same period to 6.2 million.
The average completed family size is projected to be 1.89 children by 2037, the ONS said.
The projections forecast an average UK annual population growth of 0.6% until 2037.
The ONS said a breakdown of the projected 9.6 million increase over this 25-year period was made up of 5.4 million more births than deaths (57% of the rise) and 4.2 million in net migration into the country (43%).
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But it said 5.8 million of the overall population increase (60%) was attributable either directly or indirectly to migration - including not only new arrivals in the country but the effect on the birth rate.
The figures show a sharp rise in the projected number of people aged 80 and over by 2037, from around one in 21 of the population to one in 12, the ONS said.
The projections include a "significant" increase in the number aged 100 and over from 13,000 to 111,000.
The number of people of working age for every person of state pension age is projected to fall from 3.2 to 2.7 in 2037, taking into account planned changes to eligibility for the state pension.
Richard Pereira, head of population statistics for the ONS, said: "These population projections are used across Government in terms of setting policy, they are used by the Office for Budget Responsibilty (OBR) as a key input for their long-term fiscal projections, they are used by the Department for Work and Pensions for policy on benefits and pensions, and they are used by people like the Department for Education."
The number of children aged under five is not projected to change much over the 25 years to mid-2037, the ONS said.
But the number of primary school-age children is projected to increase by 13% to reach 5.7 million. Over the same period, the number of children aged 12 to 16 is projected to rise by 10% to 4.1 million.
The population figures were seized on by Ukip European election candidate Amjad Bashir.
"Ten million more people added to the UK population in just 25 years is staggering and it could well be disastrous," he said.
"We now face a situation where we have no choice but to control immigration as the growing population is stretching our services to breaking point."
Sir Andrew Green, chairman of Migration Watch UK, said: "These latest projections confirm that we are still on course for a continued and substantial increase in population.
"By the end of the next parliament, the population will have increased by three million, the equivalent of Birmingham, Liverpool, Manchester, Glasgow, Cardiff and Aberdeen.
"This underlines the absolute need to have a net migration target and stick to it."