The population of the UK increased by more than half a million in a year, official estimates have revealed.
Natural growth – more births than deaths – of 171,800 and net international migration of 335,600 helped push the number of people living in the country to 65.1 million as of the middle of 2015.
Population change due to the difference between births and deaths is at its lowest level since the year to mid-2006.
The ONS said: “The number of births has decreased on last year’s figure and is below the average for the period, while there was an increase in the number of deaths since last year, partly attributed to flu outbreaks in early 2015.”
Meanwhile, an increase in immigration (up 53,700) and a smaller decrease in emigration (down 22,300) have both contributed to the increase in net international migration compared to that seen in the year to mid-2014.
Net international migration in the year to mid-2015 was at a similar level to that seen a decade earlier, the ONS said.
“This is caused by a combination of more people arriving to stay in the UK and fewer people leaving the UK on a long-term basis,” its report said.
International migration inflow is at its highest since the year to mid-2007, the ONS said, while outflow is at its lowest since comparable records began in 2002.
The figures indicate that migration accounted for just under two-thirds (65%) of the annual change.
In addition to the direct impact on the size of the population, 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 ONS report said.
The figures were released hours after the polls opened in the EU referendum and come after a campaign which has been dominated by debate over immigration.
A small increase of 5,800 people in the armed forces and prison populations also contributed to the growth.
England saw the biggest jump in population out of the four UK countries over the year, with a rise of 469,700, or 0.86%, to 54,786,300.
The population of Scotland increased by 25,400 (0.47%) to 5,373,000; Wales rose by 7,100 (0.23%) to 3,099,100; and Northern Ireland was up by 11,100 (0.60%) to 1,851,600.
Overall, the figures mean the number of people living in the UK has increased by around 4.7 million in a decade.
The statistics also showed that the older population continues to grow, with more than 11.6 million – 17.8% of residents – aged 65 and over, and 1.5 million (2.3%) aged 85 and over as of June last year.
Since mid-2005, the UK population aged 65 and over has increased by 21%, and the population aged 85 and over has increased by 31%.
Simon Ross, chief executive of charity Population Matters, said: “The UK population is growing faster than even the concerning trend of recent years.
“Near-record net migration and an excess of births over deaths, to which migration also contributes, are combining to keep the UK near the top of the European population growth league table.
“London and the South East, already two of Europe’s most densely populated areas, are growing denser still.”
Alp Mehmet, vice chairman of campaign group Migration Watch UK, said: “These figures confirm that our population is growing by around half a million annually – the equivalent of a city the size of Liverpool every year.
“At this rate, the UK population will increase by 10 million over the next two decades and continue to go up.
“As the population grows beyond our capability to provide for it, pressure on housing, schooling, healthcare and transport will become ever more critical. This is unacceptable to the majority of the public who wish to see net migration reduced.”