The Office for National Statistics (ONS) says the population will increase by 4.5% over the next 10 years, hitting 69.4 million in mid-2028 and passing 70 million by the middle of 2031.
The UK population was last year estimated at 66.4 million.
An ageing population is also expected to mean the proportion of people over 85 almost doubles over the next 25 years.
However, the ONS foresees a drop in the annual population growth rate – from 0.60% in mid-2019 to 0.27% by mid-2043.
Growth will slow based on assumptions that women will have fewer children because of recent falls in fertility rates, and a slower rate of increase in life expectancy.
Net international migration will account for almost three quarters of UK population growth, with the rest made up by more births than deaths, the ONS said.
Between mid-2018 and mid-2028, 5.4 million people will settle long-term in the UK, while 3.3 million go and live elsewhere.
Some 7.2 million people are expected to be born, and 6.4 million will die.
Anna Dixon, chief executive at the Centre for Ageing Better, said the figures showed the age of the population was “dramatically shifting”.
“These longer lives are a huge opportunity, but big changes are needed to our workplaces, homes, health services and communities if we are to ensure that everyone is able to age well.
“We also need to rethink our attitudes to age, and tackle the ageist attitudes which hold back too many people from enjoying a good later life.”
The ONS said: “Although migration declines at first and the number of births is stable, the number of deaths is projected to grow as those born in the baby boom after World War Two reach older ages.
“The population is increasingly ageing and this trend will continue.
“However, because of the expected rise in the state pension age to 67 years, it is projected that slightly fewer than one in five people will be of pensionable age in 2028, a similar proportion to today.”
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