The overall size of the population in mid-2020 is likely to have stood at 67.1 million, up by 284,000 on the official estimate for mid-2019, according to the Office for National Statistics (ONS).
The slowing of the growth rate in this period is largely due to the impact of the first wave of the coronavirus pandemic on mortality, the ONS said.
The figures come after it was revealed more deaths than births have been registered in the UK for the first time since 1976.
In the year to mid-2020, there were 669,000 deaths: the most in since 1986, and 13 per cent higher than in the year to mid-2019.
The number of births in mid-2020 was 701,000, the lowest number since 2003 and a continuation of the trend in fewer births seen over recent years.
ONS estimates that 622,000 people immigrated to the UK while 375,000 emigrated - making a net international migration 247,000.
There were around 11 per cent fewer internal moves than in the previous year, reflecting the impact of the pandemic on mobility from March 2020.
It is estimated that the population grew in 294 out of the 374 local authorities of the UK in the year to mid-2020, despite the effects of the pandemic and a slowing of national population growth.
Neil Park, from the population estimates unit at ONS, said: "The UK's population continues to grow, albeit at the slowest rate in two decades, surpassing 67 million for the first time. However, the 12 months to June 2020 can be broken into two clear parts.
"The first eight months, when births, deaths and migration patterns were similar to trends seen in recent years, and the four months from March, when the first wave of coronavirus hit. Some of the clearest impacts of the pandemic can be seen in the increase in the number of deaths and reduction in the number of moves made within the UK."