Immigration impact 'underestimated' on UK population, says report It was 1.35 million higher than the Office for National Statistics" (ONS) estimate, which said migration was responsible for 57 per cent of Britain's increase in population over the period. Sir Andrew Green, the MigrationWatch chairman, said: "This is about population growth, not about citizenship. "Those born in the UK to settled immigrant parents are British citizens, irrespective of their parents" country of birth. That said, it is now undeniable that the massive scale of net migration has been the main cause of our population growth and that, in the future, our population growth is likely to be almost entirely due to migration. The conventional official statistics published in the ONS official statistics do not make this clear. Drawing on ONS data, the think-tank's research suggested that even with zero net migration - the number of new arrivals minus those emigrating - the population will grow until 2041 largely due to the number of migrants who have already settled here. After that, the population would decline gradually to current levels towards the end of the century. However, with net migration running at the ONS's estimate of 165,000 a year, the population will soar to 75 million by 2041 and nearly 90 million by the end of the century. The ONS uses a modest estimate of net migration for its projections, which means population growth could be far higher in reality. In the 12 months to the end of March net migration was 243,000. Sir Andrew said: "The reality is that even if net migration is bought down to 165,000 a year, we will, in the next 25 years, have to build the equivalent of 10 cities the size of Birmingham - amounting to almost twice the population of Scotland. This would place enormous stress on our already creaking infrastructure and on our environment and it would also change the nature of British society for ever. We must not sleep walk into one of the most significant changes in a thousand years of our island's history. An ONS spokesman said: "In accordance with United Nations international standards, ONS does not define children who are born in the UK as immigrants. MigrationWatch has used a different definition which produces a higher total.