The UK's population is set to exceed 70 million before the end of the next decade, according to the Office for National Statistics.
The UK population was estimated to stand at 65.6 million in 2016.
The ONS projections are now for a 3.6 million, or 5.5%, increase over the 10 years to 2026, with the 70m mark being passed in the middle of 2029.
This is two years later than the ONS's 2014 projection that the UK population would pass the 70m barrier in mid-2027.
The reason for the change is that since the 2014 study the assumption of net migration of 185,000 a year has been cut to 165,000 a year, while the expected number of children per woman has been cut from 1.89 to 1.84 - with growth in life expectancy trimmed for men from 84.3 to 83.4 and for women from 87.1 to 86.2.
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Andrew Nash, of the ONS Population Projections Unit, said: "Over that period 54% of growth is projected to result directly from net international migration. The other 46% is because there will be more births than deaths."
Over the decade from mid-2016 projected growth in England's population is put at 5.9%, for Northern Ireland the figure is 4.2% while for Scotland and Wales the percentages are 3.2% and 3.1% respectively.
The figures, which do not take into account any Brexit impact, puts projected UK population growth between 2015 and 2040 at 16%, compared with 10% growth for France and 4% for Germany, while Italy's population is projected to see a slight decline.
The study also says that the numbers of people aged 85 and over with have doubled from 1.6m in 2016 to 3.2m in 2041.
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Analysis, by BBC Home Affairs Editor Mark Easton
The Brexit vote has made calculating the growth of the UK's population much more difficult. The level of net migration and, indeed, the potential emigration of young, fertile residents are dependent on the kind of post-Brexit agreement that emerges.
Nevertheless, the Office for National Statistics has had a stab at it and concluded that the UK population will not grow as fast as previously thought.
In 25 years' time, it believes there will be close to 73m people in the UK - some 2m fewer than they had thought before the EU referendum. Net migration will be lower and so will the fertility rate with fewer young foreign-born women coming to live in the UK.
They also expect a slower rate of increased life expectancy than previously assumed. The impact of Brexit is dependent on the kind of deal that emerges and what happens to the UK economy as a result. Those factors will affect the numbers who wish to come and wish to leave.
The ONS thinks, annually over the next 25 years, net migration will fall by about 20,000 compared with previous forecasts. But the uncertain nature of our politics makes these kind of projections fraught - particularly for those who use the data to make decisions about building roads and houses and transport infrastructure.