Deaths will consistently outnumber births in Britain from the middle of the next decade, according to the Office for Budget Responsibility (OBR), in a tipping point that will leave the country dependent on migration.

The population of people born in Britain is forecast to begin declining in 2035, according to OBR projections published on Thursday. It follows a long-term fall in birth rates.

The UK’s birth rate has fallen from three per woman on average in the 1960s to around two a decade ago and 1.5 in 2022, following a similar pattern to much of the rich world.

The country needs a birth rate of 2.1 births per woman if the population is to remain stable without migration, the OBR said.

Deaths outnumbered births for the first time since 1976 in 2020, a year disrupted by the pandemic, and figures published by the Office for National Statics (ONS) show there were just 400 more births than deaths last year.

The OBR expects a tipping point in the middle of the next decade at which point deaths consistently outnumber births. If net migration fell to zero, the population would start shrinking from 2035 as a result, dropping by 11m by the 2070s.

It poses a critical threat to the economy and the public finances as the drop in births leaves an increasingly elderly population more and more reliant on a smaller share of workers to pay their pensions and cover the cost of healthcare.

While the UK-born population is forecast to be in decline by the middle of the next decade, the OBR does not expect the overall population to fall as strong net migration flows are expected to continue.

The spending watchdog, which made the projections as part of its modelling of public finances, forecast the population will grow from 68m last year to 82m in the 2070s, on the assumption that net migration averages 315,000 people per year.

A net 685,000 people moved to Britain in 2023, down from 764,000 in the previous year but still up considerably higher than the 184,000 who came in 2019.

The previous Conservative Government repeatedly failed to meet its promises to bring down migration levels. Sir Keir Starmer pledged to reduce net migration in the Labour manifesto ahead of July’s election.

Even assuming migration into Britain continues, the declining birth rate means over-65s will account for a growing share of the population while the proportion of children in the UK will shrink.

In 1974, one in four people in Britain were aged under 16. Today one-in-five are children. By 2074, that will fall to 15pc, meaning fewer than one-in-six will be under 16.

At the same time, the share of the population aged over-65 will increase from 18pc now to 27pc in the 2070s – meaning more than one-in-four people will be on or over the cusp of retirement.

David Miles, a top official at the OBR, said the solution to the public finance challenge posed by the ageing population may be to work longer, rather than encouraging women to have more children.

He said: “People worry that with relatively low fertility rates, there’ll be a shortage of workers: who’s going to do all the work? Who’s going to look after the young and the old? I think part of the answer for that is the old themselves.

“If there is a shortage of labour, there will be natural adjustments in terms of wages and also making work more attractive and feasible for people who, at the moment, we might think of as retirement age,” he said, suggesting more flexible work would help keep older people in the workforce for longer.

“I think those adjustments over the long run I think will be very powerful [and] are probably more likely to be the answer to low fertility than trying to encourage women to have lots more children.”

Birth rates are falling across much of the world, leading to fundamental changes in societies. This is epitomised by China, the world’s second-largest economy. The United Nations expects the Chinese population to drop by more than half over the course of this century, falling from 1.4bn today to under 650m by 2100.

Decades of the one-child policy, which forbade couples from having more than one child, forcibly accelerated the country’s ageing. Despite officially cancelling the policy in 2015, the country’s population fell in both 2022 and 2023.