One in 12 people in the UK was born overseas, a population study shows.
Half of migrants are aged between 25 and 44, the ONS study shows
The Office for National Statistics data showed the overseas-born population rose more from 1991 to 2001 than in preceding post-war decades.
And the report said migration was playing an "increasingly important" role in population growth. The UK population stands at 59.2m.
But according to the ONS quarterly review, Population Trends, half of all migrants left within five years.
In its latest study, entitled Why is the UK Population Growing?, the ONS revealed around half of international migrants were aged between 25 and 44.
The overseas-born population, which stood at 4.9m in 2001, was more concentrated in the working-age group than the UK-born population.
There were 4.6 million people from non-white ethnic groups in the UK in 2001, making up 7.9% of the population.
Half of migrants are aged 25 to 44
7.9% of UK population are from non-white ethnic groups
The median age is 38.2 years, up from 34.1 in 1971
Indians accounted for the largest of these groups, followed by Pakistanis.
Immigration was just one factor which had contributed to the UK's rising population, which in 2002 hit 59.2 million - up 2.9% on the previous decade.
The annual number of births continued to exceed the number of deaths in the UK - a trend which was set to continue until 2031, according to the report.
But the population was ageing, with a declining number of under 16-year-olds and a growing proportion aged 65 and over, the report continued.
The median age in the UK rose from 34.1 years in 1971 to 38.2 in 2002. It was projected to rise to 43.3 by 2031.
Women were continuing to give birth to their first child later in life.
In 2002 the average age for having a first child was 27.3 years compared with 26 in 1992.
The report was compiled using data from the 2001 Census and other, more up-to-date sources where available, the ONS said.