The UK's population will pass the 70 million mark in less than a decade, according to official projections.
Demographers expect the landmark to be reached by 2026 as the number of people living in the country increases "steadily".
Natural growth - more births than deaths - and net migration helped push the estimated UK population to a record 65.1 million in 2015, a rise of more than half a million on the previous year.
Experts also say the growth in the size of the population is partly because it is ageing - with one in five expected to be aged 65 or over by the middle of the next decade.
In an overview of the latest trends published on Friday, the Office for National Statistics (ONS) said growth slowed during the 1970s after the "baby boom" of the previous decade, before picking up again in the late 1980s.
Recent uplifts have "generally coincided with an increase in the number of countries holding EU membership", the report added.
A growing proportion of UK inhabitants are aged at least 65, with the percentage in this age group rising from 14.1% in 1975 to 17.8% in 2015.
Over the same period, the proportion of children aged 15 and younger has declined from over 24% to less than 20%.
It is forecast that a fifth (20.2%) of the population will be aged 65 and over in 2025, rising to a quarter (24.6%) in 2045.
Natural change has had an impact on the number of occupants. Since 1955, the number of births in the UK has been higher than the number of deaths in every year bar 1976.
The rise in the population since the 1990s has also been attributed to the growth of net migration - the number of people arriving to live in the UK minus the number departing.
Statisticians said the direct effect of net migration increased the population by more than 250,000 people per year on average from 2004 to 2015. This is about 50,000 more people per year than natural change for the same period.
Current and past international migration also has "indirect effects" on the size of the population as it changes the numbers of births and deaths in the UK, the report added.
Immigration has been higher than emigration since the early 1990s. In 2015, the inflow was 631,500 - more than double emigration at 299,200.
The report said rises in immigration "have tended to coincide with the expansion of the EU, allowing more people to freely migrate to the UK".
In 2004, eight central and eastern European countries - Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia - joined the bloc, while Romania and Bulgaria became member states in 2007.
The UK is currently estimated to have the third largest population out of the EU member states plus Iceland, Liechtenstein, Norway and Switzerland.
But it is projected the surpass Germany and France to become the most populous of the 32 nations at 77 million by 2050.
On the consequences of the population changes, the ONS report said: "While living longer is a cause for celebration, an ageing population may result in fewer people of working age to support those of pension age.
"While a larger population increases the size and productive capacity of the workforce, it also increases pressure and demand for services such as education, healthcare and housing."