The number of people over 65 is projected to increase to 23%
The population of the UK will rise from 61m to 71.6m by 2033 if current trends in growth continue, the Office for National Statistics (ONS) has said.
Just over two-thirds of the increase is likely to be related directly or indirectly to migration to the UK.
If the projected increase materialises, the population will have grown at its fastest rate in a century.
But one think tank said the projections were based on trends over the past few years that may not continue.
National population projections are produced every two years to provide an estimate of future population which is used for government planning for pensions and the welfare state.
The latest figures show that if current trends continue:
- The population will grow by more than 10m by 2029, less than half the time it took to rise from 50m to 60m between 1948 and 2005
- The population of pensionable age will rise by 32% over the next 25 years to 15.6m, with the number aged over 85 more than doubling to 3.3m
- In 2033, there will be 2.8 people of working age for every person of state pensionable age - a fall from 3.2 in 2008
- By 2033, the population of England will be almost as large as the current populations of England, Scotland, Wales and Northern Ireland combined.
The ONS figures suggest that 180,000 new immigrants will arrive every year for the next 25 years - and will have both a direct impact themselves on the population, and an indirect one if they go on to have children.
But that number for new arrivals is 10,000 a year lower than the last ONS projection two years ago and immigration minister Phil Woolas said that showed the government's points-based system was working.
"Today's projections show that population growth is starting to slow down, the impacts of the radical reforms we have made to the immigration system over the last two years are working," he said.
However, MPs Nicholas Soames and Frank Field, chairmen of the cross party group for balanced migration, said the UK was "on course for an unsustainable and unacceptable rise in population".
"Over the next Parliament, at a time of public spending cuts, the government will have to find the money to pay for one million new immigrants - a city the size of Birmingham."
They called on all major parties to promise "firm and effective" measures to keep the population below 65m.
"If politicians want to rebuild trust and counter extremism, they must stop ignoring the public's deep concern about this," they said.
Shadow immigration minister Damian Green said a Conservative government would introduce a limit on the numbers allowed to come to the UK to work, "as well as other measures to fight illegal immigration".
Sir Andrew Green, chairman of think tank Migration Watch UK, said the government was "in denial" and "really serious measures" were needed to bring immigration down.
"70% of the fastest growth rate in history is due to immigration," he said. "That is equivalent to the entire population of London in the next 25 years.
"The government's own claim for the effect of their recent measures is an annual reduction of 20,000. Today's projections show that net immigration must be reduced by 180,000 a year if we are to hold the UK's population at 65 million."
Impact of recession
Guy Goodwin, ONS director of population statistics, stressed that the figures were not forecasts or predictions and did not "take account of new or future policy initiatives".
"Really, they're just a benchmark that policy-makers and politicians can look at and say, 'this is where we are heading if things continue very much as they are'."
Tim Finch, from the Institute for Public Policy Research, says there is also a positive aspect to inward migration.
"If you have a lot of migrants in your society, the chances are they're going to be contributing to the tax base, which allows you to support an ageing population," he said.
Campaigners for sustainable development said the ONS figures were a "wake-up call for politicians".
Peter Madden, chief executive of Forum for the Future, warned of the impact of public services, housing and the environment if the projected rises did materialise.