The population of the UK grew by more than 400,000 last year, according to the latest official figures.
The Office for National Statistics estimated there were 64.1 million people in the UK in June 2013, a rise of 0.63% on the previous year.
Just over half of the growth was accounted for by natural change - births minus deaths - while net migration represented 46% of the rise.
A quarter of the UK population growth was in London.
The Office for National Statistics (ONS) said the British population grew more last year than in any other EU country.
While there were 212,100 more births than deaths, the figures show 183,400 more immigrants arriving in the UK than emigrants leaving.
Last year's growth was slightly below the average seen over the previous decade.
The estimated population increased in England by 0.7% to 53.9 million, in Scotland by 0.27% to 5.3 million, in Wales by 0.27% to 3.1 million, and in Northern Ireland by 0.33% to 1.8 million.
The estimates showed the population of the UK had risen by more than the average seen across the European Union, exceeding the growth rate in its four most populous member states.
The ONS said the UK population had increased by about five million since 2001 and by more than 10 million since 1964.