The estimated population of the UK has hit 66.8 million after growing at the slowest rate for 15 years, official figures show.

According to data released by the Office for National Statistics (ONS) on Wednesday, there was an estimated 66,796,807 people living in the country at the end of June last year.

The population growth rate over the 12 months to the middle of 2019 was 0.5%, the slowest since mid-2004, the ONS said.

It said decreasing numbers of births and net international migration has contributed to the growth rate figure.

The year to mid-2019 saw the fewest births since mid-2005, at 722,000.

The ONS said net international migration of 231,000 people was 44,000 fewer than in the year to mid-2018.

This change was a result of 17,000 fewer immigrants arriving than in the previous year (a 3% decrease) and 28,000 more emigrants (an 8% increase).

Meanwhile, there were 593,000 deaths in the year to mid-2019, 5% fewer than in the previous year.

Neil Park, from the ONS population estimates unit, said: “The population grew at the slowest rate for 15 years between mid-2018 and mid-2019.

“This is due to the lowest number of births for 14 years alongside an increase in emigration and a fall in international immigration.

“The figures we’re publishing today highlight the variation in the population across the UK.

“For example, the population density in London is 24 times higher than that for the South West of England.

“Also, the proportion of people aged 65 or over ranges from over 30% in coastal areas such as North Norfolk to less than 8% in parts of central London like Tower Hamlets.”

According to the ONS, in mid-2019, there were an estimated 12.4 million people aged 65 years and over – 18.5% of the population – while 2.5% of people were aged 85 or older.

The highest proportions of older people were most commonly found in coastal areas of southern and eastern England, the ONS said.

The country’s age groups are changing at sharply different rates, figures also suggest.

While the number of children (those aged under six) increased by 8% between mid-2009 and mid-2019, the number of people aged 65 and over went up by 23%, while those aged 70 and over jumped by 25%.

The working age population – those 16 to 64 years – showed the lowest growth of any age group over this period, rising by just 3%.

The average (median) age for the population of the UK in mid-2019 was 40.3 years – one year higher than it was in mid-2009.

The UK’s population also appears to have an uneven spread, with the density ranging from 5,700 people per square kilometre across London to fewer than 50 people per square kilometre in the UK’s most rural local authorities, the ONS said.

When looking at local authority areas across the UK, the largest population growth from mid-2018 to mid-2019 was recorded for the City of London (up 11.7%), followed by Camden (3.0%) then South Derbyshire, Dartford and Tewkesbury (all 2.6%).

The ONS data also revealed that Northern Ireland saw the fastest population growth of the four nations of the UK, rising by 0.64% from mid-2018 to mid-2019.

Meanwhile, England saw estimated growth of 0.55%, followed by Scotland (0.46%) and Wales (0.45%).

It is the first time since the year to mid-2009 that Northern Ireland was the fastest-growing nation in the UK, with a younger population driving a comparatively high level of natural change leading to a higher number of births and a lower number of deaths.

Robin Maynard, director of charity Population Matters, said: “These are welcome figures, especially the decline in birth rate, which appears to confirm that people are choosing smaller families.

“After a steady rise in the early 2000s, birth rate has almost halved since 2012, and that means a better future for those children being born.

“We should never see population as just a number – more people, consuming more stuff, lessens our quality of life, produces more emissions, uses more of the Earth’s resources, and puts more pressure on our threatened natural environment.”

Louise Ansari, director of communications and influencing at the charity Centre for Ageing Better, said the country could expect to see increasing numbers of over 65-year-olds.

She added: “That means as we look to life beyond lockdown, it’s vital that government and society as a whole grasps the urgency of adapting to this dramatic shift in the age of the population.

“We need to futureproof our housing, our health systems, our workplaces and our communities.”