ONS predicts 54% of growth in next decade will be from net international migration and 46% from more births than death.
Demographers project that the population will rise by 3.6 million, or 5.5%, over the next 10 years, rising from an estimated 65.6 million last year to 69.2 million in mid-2026.
The Office for National Statistics said the population was projected to pass 70 million by mid-2029, reaching 72.9 million in 2041.
This is two years later than the ONS projection in 2014, which stated the UK population would pass the 70 million barrier in mid-2027.
There are three main reasons behind the revised rate, according to the ONS. First, since the 2014 study the assumption of net immigration of 185,000 a year has been lowered to 165,000. Second, the predicted number of children per woman has been reduced from 1.89 to 1.84. And third, the projected rate of increase in life expectancy has been cut for men from 84.3 to 83.4 and for women from 87.1 to 86.2.