The Office for National Statistics (ONS) said the UK was set to reach the milestone by mid-2027.
Within 25 years, the UK population is set to swell from 62.3 million last year to 73.2 million, with more than two thirds of this increase a result of immigration, the figures showed.
Of the 10.9 million increase, 5.1 million (47%) is due to projected net migration, 2.3 million (21%) from migrant women giving birth, and 5.8 million (53%) due to projected natural increase as the number of births outpace deaths.
Officials said the UK's population was projected to rise by 438,000 a year until 2035 - the equivalent of adding a city the size of Bristol every year.
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.
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.
"Much has been done, but there is more to do to bring down net migration to the order of tens of thousands a year and ensure migration which benefits the UK."
Sir Andrew Green, chairman of the campaign group Migration Watch UK, said: "These figures confirm that the UK's dramatic rise in population will continue unabated.
"The population is now set to hit 70 million in 16 years time, over two thirds of which is due to immigration.
"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."
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.
The population is also projected to become older gradually, with the median age rising from 39.7 years in 2010 to 39.9 years in 2020 and 42.2 by 2035, the figures showed.
As the population ages, the numbers in the oldest age groups will increase the fastest.
There were 1.4 million people in the UK aged 85 and over last year, but this is expected to increase to 1.9 million by 2020 and to 3.5 million by 2035, more than doubling over 25 years.
The ONS added that the number of people aged 90 and above is on course to more than triple by 2035, the number of people age 95 and over is set to more than quadruple, and the number of centenarians is projected to rise from 13,000 in 2010 to 110,000 in 2035, a more than eightfold increase.
The large number of people born immediately after the Second World War and the baby boom of the 1960s will see the number of people of state pension age increase by more than a quarter, from 12.2 million to 15.6 million by 2035, the ONS said.
This will come despite the changes which will see the state pension age rise to 65 for both sexes by 2020 and then increase in three stages to 68 years for both sexes between 2024 and 2046.
The size of the working-age population - those between 16 and the state pension age - will rise 16% over the same period, from 38.5 million in 2010 to 44.7 million by 2035, the figures showed.
There were 3.16 people of working age for every person of state pension age in 2010, but this is on course to rise slightly to 3.28 by 2020 and then drop - due to the changes in the state pension age - to 2.87 by 2035.
Eurostat figures also showed the UK will outstrip Germany and France to become the biggest country in the European Union by 2043, the ONS said, with the UK's population estimated to reach 74.4 million that year, more than Germany's 73.7 million.
Statisticians said this could be partially explained by lower fertility rates in Germany. Only three countries - Luxembourg, Cyprus and the Republic of Ireland - are projected to grow faster than the UK over this period.
PA
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