The UK population will grow to more than 70 million over the next 24 years, according to official predictions published today.
By the year 2031 there will be about 71 million people living in Britain, the Office for National Statistics (ONS) said.
Researchers said if past trends continue, the population will increase from about 60 million in 2006 to 65 million in 2016 and pass 70 million in 2028.
The projected figures are based on a range of factors including immigration, fertility and average lifespans.
They are the first to use data on immigration which was recently increased by the ONS.
Last month the organisation said the number of people migrating to the UK had increased by 45,000 a year to 190,000.
England will grow the fastest with an increase of 8% by 2016, compared to 7% in Northern Ireland, 5% in Wales and just 3% in Scotland, the ONS said.
Officials said long-term assumptions of future fertility, life expectancy and migration are all higher than those made in the previous projections.
Researchers have increased their migration estimates from a net increase of 145,000 or more per year in 2004 to 190,000 or more each year.
They also believe almost half of the 4.4 million population increase in the UK over the next decade will be fuelled by migration.
The study revealed the changing structure of Britain's ageing society as people live longer.
Researchers said the average age is expected to rise from 39.6 in 2006 to 40.6 in 2016 and to 42.6 by 2031.
The number of people of state pensionable age is projected to increase by 7.2% from 11.3 million in 2006 to 12.2 million in 2010.
Researchers said that this year, for the first time ever, the population of people claiming state pensions will exceed the number of children.
By 2016, there will be 400,000 more people claiming state pensions than children, rising to more than two million in 2031.
In 2006 there were 3.3 people of working age for every person on a pension, but this ratio will slowly decline.
Despite the forthcoming change of the age at which women can claim state benefits, it will decline to 2.9 by 2031.
The ONS said the figures are based on demographic trends and cannot predict the impact of future Government policies or changing economic circumstances.
A spokesman said: "They simply provide the population levels and age structure that would result if the assumptions about fertility, mortality and migration levels were to be realised in practice."Reuse content