The UK population increased to an estimated 68.3 million and the main driver was net immigration, the Office for National Statistics (ONS) has said.

Some 68,265,200 people were resident in the UK to the middle of last year, up 662,400 from 67,602,800 in mid-2022 -- which itself is also the biggest annual numerical increase since 1971.

In the year to June 2023, 1,185,600 people immigrated to the UK and 508,300 people left the country, leaving net international immigration -- those entering minus those leaving -- at 677,300, the figure being what the ONS described as the "main contributor to population increase" across all four countries.

They added that with the exception of the COVID-19 pandemic year of 2020, this was the first time since 1976 that there had been natural negative change in the population.

Professor Sarah Harper, director of the Oxford Institute of Population Ageing, said the negative natural change is "not unexpected."

Demographers say a TFR of around 2.1 is the minimum needed to maintain a population, and some concerned about fertility have warned that that decrease could accelerate, owing to changing social attitudes among younger people.

Also at ARC, economist Philip Pilkington and demographer Paul Moreland warned that their modelling suggests fertility could "fall[] off a cliff" with younger generations having very few children, noting that their "lifestyle and their aspirations do not correlate with having large numbers of children."

Morland and Pilkington said that the UK must choose the option of "more children," or accept mass immigration or economic stagnation.

Labour said before winning the election that the overall level of net immigration "must be properly controlled and managed," pledging to reform the points-based system -- but would not commit to an annual cap.

Responding to the figures, the prime minister's official spokesman said Sir Keir Starmer is determined to bring down net immigration.

Downing Street said that Starmer and Home Secretary Yvette Cooper "have been clear that overall net migration does need to come down, and we should end the situation where legal migration is used as an alternative to tackling skill shortages in the UK."

The spokesman added that the Migration Advisory Committee is conducting assessments to highlight key sectors where labour market failures have led to high overseas recruitment and that "rules around migrant sponsorship will be toughened to ensure employers guilty of flouting employment laws are banned from hiring from abroad."