The population of the UK will be the largest in Europe by 2050, overtaking France and Germany, mainly due to an ageing population and immigration.
The UK has seen more births than deaths in every year since 1976, helping drive the population to 65m, according to figures released by the Office for National Statistics.
This figure will soar to 76m in 2045.
A quarter of the population will be over 65 in three decades
Increasing life expectancy is one of the main reasons for the population increase - with people living longer due to improved healthcare and lifestyles. Girls born in 2015 can expect to live 82.8 years from birth, while boys will live to 79.1 years.
This has meant that the proportion of people aged over 65 has increased from 14.1 per cent in 1975 to 17.8 per cent in 2015 - and is projected to expand further to almost a quarter of the population by 2045.
It is estimated that there will be an extra 10.8m over 65s in 2045 than there were in 1975 - accounting for over half of the population growth seen in that period.
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
This growth will help the UK have a larger population than any European Union country by 2050, at 77 million, according to Eurostat. It is currently the third biggest country, behind Germany and France.
According to the ONS, while the proportion of Britons at "traditional working age" has remained stable over the last 40 years, it is projected to fall in future years as a result of the growth of the ageing population.
Immigration has increased the population by 250,000 per year
Aside from natural change, immigration has also helped increase the population since the 1990s. In 2015, 631,500 people immigrated to the UK, while emigration was half its levels at 299,200.
The growth of net migration has 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.
As well as the direct impact of people migrating to the UK, immigration also impacts the population due to past movements impacting the numbers of births and deaths in the country.
What does this population growth mean?
A growing population is good for the economy as it can mean that there are more people available to participate in the workforce.
But an ageing population can also lead to strains on the NHS and other public services in the country. There were estimated to be 308 of a pensionable age for every 1,000 people of a working age in 2016 - and this figure is projected to increase to 365 in 2037.
When releasing the data, the ONS 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.
"This increase means that there will be fewer people of working age to support a larger population over State 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."