Britain’s population grew by 491,000 in the year to mid-2014 to nearly 64.6 million, with net migration accounting for 53% of the rise and births 46%

The UK population is at a record high, official figures show, with net migration having overtaken natural population change to become the largest contributor to growth.

The Office for National Statistics said 582,600 migrants arrived in the UK between mid-2013 and mid-2014, compared with 322,900 people who left the country.

An ONS spokesman said the statistics meant the UK population had grown by 491,100 since2013, a 0.77% rise to 64,596,800. This is marginally higher than the annual increase of 0.75% seen over the last decade.

The last year in which net migration was larger than natural population growth – the balance of births and deaths – was 2011.

The number of births in the year to mid-2014 was down compared with the previous year, continuing the downward trend seen since 2012, an indication that last decade’s baby boom could be slowing.

The “natural growth” of the population was 226,200 (777,400 births minus 551,200 deaths) in the 12 months to mid-2014. Births accounted for 46% of the population growth, but migration accounted for 53%, the widest gap between the two figures since 2008. The other 1% growth was the result other changes and adjustments, mainly in the armed forces.

Carlos Vargas-Silva, a senior researcher at Oxford University’s migration observatory, said the figures were not surprising. “Over the previous three years net migration decreased compared to the pre-recession levels. That was an exception. But the numbers of net migration and natural change can go back and forth. While this year net migration is the biggest contributor to population growth, next year it might be different.”

He also pointed out that the net migration figure did reflect the overall impact of migration in the UK. “This is not a holding pack of migration, it is net migration. When people come to this country they will have children and that will also contribute to population growth, but it will be part of the natural change component.

“Within the EU, people are coming to the UK for employment. It’s a combination of the push factor – high EU unemployment – and the pull factor – a strong UK economy and availability of low-skilled jobs. Outside of the EU, study has been a key factor.”

The statistics also reveal that the number of deaths in the UK has fallen since last year and is at its lowest for more than 50 years. The median age of the UK population now stands at 40, the highest ever estimated.

There are more than 11.4 million people aged 65 and over, up from 11.1 million the previous year. The number of men aged 85 and over has increased by 61.8% over the past decade, compared with an increase of 25.9% for women, driven largely by changes in smoking habits and advances in the treatment of circulatory illnesses.

Population growth was the greatest in southern and eastern England, at 1.08% and 0.92% respectively. London had the highest growth, with population in the capital up 1.45%, a result of 82,400 more births than deaths and the highest net international migration of all regions at 107,400.

London also continued its pattern of having the greatest outflow of people to other parts of the UK , with more people of every age group except 21 to 28-year-olds leaving the city.

The local authority with the greatest population rise was the City of London, up 5.54%, followed by Tower Hamlets up 4.08% and Westminster up 2.84%, all the result of migration. Forest Heath came fourth because of an increase in the number of armed forces in the area.

The estimated population of England as of mid-2014 was 54.3 million, up 0.84%; Scotland was 5.3 million, up 0.37%; Wales was 3.1 million, up 0.31%; and Northern Ireland was 1.8 million, up 0.59%. No country of the UK or region of England witnessed population decline.

Simon Ross, the chief executive of Population Matters, the UK’s leading charity working on population and sustainability issues, said the ONS estimates were a warning of the urgency of attempting to to reduce population growth.

“We are all affected adversely by the rapid population growth of recent decades,” Ross said. “Examples include pressure on housing and public services to the environment and climate change. It’s time we addressed the population problem, and find ways to live sustainably and happily in the long-term.

“As a nation we need to invest more in addressing these issues. Our emphasis should be on improved family planning and women’s education and empowerment, together with public information campaigns about the immense strains population and consumption growth place on our planet.”

• This article was amended on 29 June 2015. An earlier version said “There were 226,200 births last year, a quarter of which were to mothers born outside the UK.” In fact 226,200 is the “natural growth” of the population from mid-2013 to mid-2014, and about a quarter of the births in the UK in 2013 were to mothers born outside the UK.