The United Kingdom’s population is likely to grow to 71.0 million by mid-2034, below a previous projection of 72.2 million due to lower immigration assumptions, the country’s statistics office said on Tuesday.Net immigration looks set to add 2.2 million people to the UK’s population of 69.3 million between mid-2024 and mid-2034, while there would be a drag of 450,000 from deaths exceeding births, the Office for National Statistics said.Net migration was likely to remain the only source of expected population growth, the ONS said.The lower expected population growth could add to the fiscal headwinds that will face finance minister Rachel Reeves in her autumn budget.However, the agency said the projections should not be treated as a definitive forecast as they are largely based on an extrapolation of recent trends and do not attempt to predict future changes in government policy or other factors driving migration, fertility or mortality.Tuesday’s projection showed the extent to which Britain’s population is ageing. The number of pensioners is expected to rise by 15% to 14.2 million between 2024 and 2034, while the number of children aged under 16 will fall by 13%.The ONS also trimmed its assumption for fertility to 1.42 children per woman from 1.45 in its last projections made two years ago.There was a more substantial cut to the ONS’ assumption for future rates of net immigration to 2,30,000 a year from 3,40,000 in the 2024 report.”This reflects the sharp decrease in net migration since it peaked in 2023, as well as expert advice,” it said.Britain saw immigration surge after the COVID-19 pandemic, reflecting pent-up demand as well as a decision by former Prime Minister Boris Johnson’s government to relax rules particularly for low-paid care workers and their families.After net immigration approached 1 million in the 12 months to June 2023, rules were tightened and it fell to 204,000 in the 12 months to June 2025.The population of England is expected to grow faster than elsewhere in the UK. England’s population is projected to rise by 2.9% by mid-2034, compared to growth of 1.0% in Wales, 0.6% in Northern Ireland and 0.3% in Scotland.