LONDON - Britain's population has officially passed the 66 million mark, the Office for National Statistics (ONS) said Thursday.

In the year ending June, 2017, an extra 392,000 people were added to the total population of England, Scotland, Wales and Northern Ireland, reaching 66,040,229. But ONS said the 0.6 percent growth over the year was the lowest since mid-2004.

The year ending June 2016 saw the population rise by 538,000 which was the highest population growth since 1948.

ONS said 41 percent of the population growth occurred from natural change (births minus deaths) and 59 percent through net international migration.

The reduction in the number of immigrants was the largest single driver of the lower level of population growth in the year to mid-2017. Immigration in the year to mid-2017 was 572,000, but in the same year 342,00 people left the country, 28,000 more than the previous year, representing a 9 percent increase in the number of emigrating. ONS said the EU Brexit referendum is likely to be one of the key drivers for the figures.

ONS said the largest inflow of immigrants to the UK was from Romania (50,000) followed by China, India, France and Poland.

The new statistics show 12 million UK residents were aged 65 and over in mid-2017, or 18.2 percent of the population, with the large 1947-born cohort now being aged 70

A decrease in net international migration in mid-2017 has affected the rate of population growth in some places more than others, with England's growth rate decreasing more than the other regions of the UK to 0.64 percent, and London's rate nearly halving to 0.63 percent

According Statistician Neil Park, head of Population Estimates Unit at ONS, this is the lowest annual population growth since 2004 due to a fall in net migration, fewer births and more deaths than previously seen.