The UK population is projected to increase by three million over the next decade.
It would take the estimated population from 66.4 million in mid-2018 to 69.4 million in mid-2028, according to the Office for National Statistics.
This means the population is projected to pass 70 million by mid-2031, reaching 72.4 million by mid-2043.
But, according to the projections, which are published every two years, the UK population growth rate is slower than in the projections made in 2016, with the expected population anticipated to be 0.4 million less in mid-2028 and 0.9 million less in mid-2043.
There is expected to be a growing number of older people, with the proportion aged 85 and over projected to almost double over the next 25 years.
Figures released earlier this year showed the number of people living in the country rose 0.6% in the 12 months to June 2018.
The rate is the same as the year before and remains the slowest since mid-2004.
London saw the highest population increase, while Scotland saw the lowest, the Office for National Statistics data shows.
Neil Park, of the ONS, said: “For the fifth year in a row, net international migration was a bigger driver of population change than births and deaths.
“Overall population change to the year mid-2018 has remained fairly stable as an increase in net international migration has been roughly matched by the fewest births in over a decade and the highest number of deaths since the turn of the century.”
Net international migration over the 12 months was 275,000, which was “broadly in line” with the average of the last five years.
There were 744,000 births in the 12 months – the fewest in any year since 2006. Deaths rose 3% to 623,000, the most since 2000.
This means natural population change – the number of births minus the number of deaths – is now at its lowest since 2004.
Laura Gardiner, of the Resolution Foundation think tank, said: “The UK population is ageing particularly fast at the moment because rising life expectancy is being amplified by the large baby boomer generation moving from working age into retirement.
"This has huge implications for public service provision across the UK, and how those services are funded."
