The UK population has seen its sharpest annual rise in nearly 70 years, with the nation expected to hit 70 million people and London 10 million residents within a decade, official figures revealed on Thursday.

Demographers at the Office for National Statistics (ONS) calculated there were an estimated 65.6 million people living in the UK at the end of June last year.

Over the previous 12 months, Britain saw the number of inhabitants rise by 0.8 per cent or, in numerical terms, 530,000 people — the biggest increase since 1947 when resident numbers went up by 551,000 during the postwar baby-boom.

Largest driver of mid-2016 population growth was net migration (62%); but births increased, deaths decreased https://t.co/dx4DWIrWpy pic.twitter.com/sOyLMUhh78 — Rich Pereira (@RichPereira_ONS) June 22, 2017

The ONS noted they recorded a rise in births and fewer deaths in the year to June 2016, but said the main driver of population growth is still net migration, which represented 62.4 per cent of the total increase.

Official population projections drawn up last year, to help councils and the NHS plan ahead with regards to providing services, showed the population of England is set to soar by more than four million people in the next decade.

The ONS expects immigration to account for almost half of the expected population growth, which the statistics body notes will push the number of London residents past the 10 million mark for the first time.

While London’s growth rate was significantly higher than the rest of the country, it was also unusual with respect to national internal migration. Although the population of London grew significantly in the past year overall, it also saw more people moving away from the region to other parts of the United Kingdom than any other region.

David Wood, who headed immigration enforcement at the Home Office until 2015, on Friday disclosed that there are more than 1.2 million illegal immigrants currently living in Britain, and warned border controls are ineffective.

Home Office estimates for illegal immigrant numbers have been kept secret since 2005 to save ministers from “embarrassment”, according to experts.

But in a paper for Civitas, written with former Home Office speechwriter Alasdair Palmer, Mr. Wood revealed the Home Office believes that each year between 150,000 and 250,000 foreign nationals who are supposed to return to their home countries fail to do so.

The figure includes people who break into Britain illegally in the back of trucks, those who are refused asylum but don’t leave (around 26,000 last year), and visa overstayers including students and others.

Mr. Wood and Mr. Palmer urged ministers to get better at deporting people who should not be in the UK, and called for officials to start using National Insurance numbers to estimate immigration, noting NI numbers showed an extra 827,000 foreign citizen registrations in 2015 whilst official figures put the number of newcomers to Britain at 550,000.