Average life expectancy will rise by three years
Figures produced by the Office for National Statistics (ONS) predict the net number of immigrants coming into the country by 2025 will be around 135,000 a year.
These numbers will provide a boost to the overall UK population - last year an estimated 59.8 million - which is expected to grow to nearly 65 million by 2025.
Britons can also look forward to a longer life, as the survey points to an increase in life expectancy for men and women of about three years.
The projections, produced every two years, serve to guide government policy in areas such as the NHS, immigration, benefits and pensions.
Fewer children
Government Actuary Chris Shaw has drawn up the projections based on assumptions on mortality, fertility and migration rates.
In 1998 the ONS predicted 95,000 immigrants would enter the UK each year but this figure was reviewed and raised by an extra 30,000 for this year.
The report predicts a jump in life expectancy from 75.5 to 78.9 for men, and from 80.3 to 83.2 for women.
The ever-shrinking family, which has been gradually falling from a peak of 2.5 children in the 1930s, is expected to level off to a fertility rate of 1.74, according to the survey.
Longer-term projections expect the population to edge up by only another million between 2050 and 2040 - peaking at around 66 million before then starting to fall.
"The population will peak and start to drop off due to the number of deaths in the larger groups of people born after the Second World War and during the baby boom of the 60s," said Mr Shaw.
There will also be differences in the rate at which population numbers will rise and fall around the UK.
The English population will continue to rise steadily, while there will be a gradual drop in Scotland, with Wales and Northern Ireland rates peaking in the next 30 years before then starting to fall.
Getting older
Britain's population will continue to age over the next 25 years, with the average age expected to rise from last year's 38.8 up to 42.6.
The workforce will be able to draw on more people of working age in the future, with numbers expected to rise by 6% to 39 million by 2011.
A further boost in females of working age will be provided when the planned change in women's state pension age rises from 60 to 65 sometime between 2010 and 2020.
The next projection figures, to come out in 2003, will take into account the 2001 national population census findings.
