The population of the UK is on course to reach 70 million over the next 16 years, official figures suggest. The Office for National Statistics (ONS) said it was set to rise from 62.3 million in 2010 to 67.2 million by 2020 and 73.2 million by 2035.
It says just over two-thirds of the projected increase from 2010 to 2035 is either directly or indirectly due to migration.
This will be due to people entering the UK, and also their future offspring.
The ONS statistics say this change may be 47% directly attributable to future migration and a further 21% indirectly due to future migrants' children being born in the country.
The effect of immigration is also expected to have an increasing impact.
Some 2.1 million (44%) of the increase over the next 10 years could be down to net migration, the ONS added.
The remaining 2.8 million (56%) is a projected natural increase as a result of births outstripping deaths.
Projections for net immigration have been revised up by 20,000 to 200,000 a year since the last estimates two years ago.
The projected UK population by 2020 is 0.7 million (1%) higher than the statistics suggested two years ago.
The average age of people in the UK is also set to rise from 39.7 years in 2010 to 39.9 years in 2020 and 42.2 years by 2035.
The number aged over 85 is expected more than double over 25 years from 1.4 million in 2010 to 1.9 million by 2020 and 3.5 million by 2035.
Those people aged 90 and above is projected to more than triple by 2035, and the number aged at least 95 is projected to more than quadruple.
The government has said it wants to cut non-EU immigration from about 200,000 to "tens of thousands" by 2015 through an immigration cap.
Immigration Minister Damian Green said: "Immigration to the UK has been too high.
"That's why we have made sweeping changes to get a grip on immigration in this country, closing down routes subject to abuse and taking action against those with no right to be here.
"As people return home this evening crammed into public transport and on congested roads, they could well ask where all of these people are going to fit."
"A managed migration policy can be a major part of the solution to the problems the UK faces."