The UK population is on course to reach 70 million within 16 years, figures showed.
The Office for National Statistics (ONS) said the UK was set to reach the milestone by mid-2027.
Within 25 years, the UK population will swell from 67.2 million last year to 73.2 million, the figures showed.
Almost half of the population increase over the next decade will be fuelled by immigration, the ONS figures showed.
Of the 4.9 million projected increase over the next 10 years, some 2.1 million (44%) is the assumed net number of migrants. The remainder, some 2.8 million or 56%, is from the projected natural increase as the number of births outpace deaths.
The impact of migration is also expected to increase.
Of the 10.9 million projected population increase by 2035, 5.1 million (47%) is due to projected net migration, while 5.8 million (53%) is due to projected natural increase.
The new long-term assumption for net migration - the number of people coming to the UK less the number leaving - is 200,000 each year, 20,000 a year more than in the last projections two years ago, the ONS report showed.
Some 15,500 of this is based on England alone, with 5,500 extra in Scotland and 500 fewer in both Wales and Northern Ireland. Overall, the projected UK population by 2020 is 0.7 million (1%) higher than in the previous projections two years ago.
"The 2010-based projections assume an extra 0.5 million births and an additional net gain of 0.3 million more migrants countered by 0.1 million more deaths," the ONS report said.